RGB and CMYK Best Color Match

[peachytoo]

I am trying to develop matching items, some in fabrics which printer requires RGB color codes, and others in paper, which printer requires CMYK color codes.  The key to this project is, I need them to match in color, or at least be a very, very, close match since they are intended to go together in a set.  I use Gimp for fabric and Photoshop for paper (conversion to pdf required).  I know how to change between codes and such, but I find every source I have looked at, none agree which number codes are a match.  Is one program, conversion chart, or any other source, the most accurate match in converting the codes that will result in matching finished products?  I have spent so much time and money trying to resolve this matching issue I am about ready to just give up.  Please help if you can.

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[Derek Cross]

Apart from certain high-end inkjet printers, all printing of colour (leaving out spot colours) is printed in CMYK. And CMYK has around a third less colours than RGB, particularly in the bright greens and bright oranges, and you’re stuck with that.

It‘s best to work in RGB in your software and you can softproof to CMYK to see the effects of conversion, and you can export to PDF/X-4 for converting to CMYK for commercial litho printing (unless your printer gives you a different spec). Ensure your system is colour managed by calibrating your monitor etc.

Presumably, you’re aware of the various colour matching systems, such as Pantone.

There’s a good online video tutorial from Lynda.com that might help (you can get a 30-day free trial):

InDesign: Color Management

[davescm]

Hi

There are some fundamentals that you need to understand or you will be chasing the end of the rainbow forever.

First, RGB colour codes and CMYK color codes are meaningless without the context of a colour space. So the colour represented by RGB 123,56,34 in sRGB is different to that represented by the same numbers in Adobe RGB, or different again to the same numbers  in ProPhoto RGB. It is the same with different CMYK spaces.

Second, hex values are no more accurate.  They are just the same RGB numbers represented in the base16 instead of base 10. Without the context of a particular colour space those numbers are meaningless.

Third, the range of colours that can represented by a particular colour space, i.e the gamut, differs. So a colour that can be represented in sRGB may not be available in a particular CMYK colour space.

Finally, properly set up colour management will convert colours for you between one space and another and allow you a degree of choice of how to handle those colours which are out of gamut.

Dave

[pasamike]

Dave, 

Is there one RGB mode that is better than another to achieve closer matches to CMYK? I'm no expert on this subject but I am about to pull what little hair I have out, trying to find an exact color match based off two Hex Codes. 

[D Fosse]

You'll be much better off if you abandon hex once and for all. It's a bad habit, because most people treat it as an absolute number, an absolute color, which it isn't. So in practice, because of the way it is used, it will only be confusing and misleading.

Numbers mean nothing until they refer to a specific color space. That applies to RGB, CMYK and hex alike.

RGB color spaces are sRGB, Adobe RGB etc. CMYK color spaces are US Web Coated SWOP, ISO Coated 300%, etc.

You always need to know which color space the numbers refer to. Always, no exception. Once you have that, the color is absolutely defined and you can match one to another perfectly.

An additional consideration is gamut. Color spaces are large or small, meaning there's a limit to how saturated a color can be in any given color space. If the color in question is outside the gamut boundary, it cannot be reproduced in that color space. There is nothing you can do about that; it's a hard limit. The only thing you can do is use a bigger color space, but that is often not possible. So this is something you have to learn to accept and deal with.

[Test Screen Name]

The key thing is that there are many web sites giving conversions between RGB and CMYK and there are ALL nonsense set up by people who don‘t understand it. Ignore them all all and learn about colour management, and you’ll soon see the sites are nonsense.

[Derek Cross]

These replies summarise some of the issues with colour matching. In many situations being "near enough" is acceptable, but in situations like yours, where you are trying to match clothing colours in catalogues accuracy can be vital, I would suggest you consider employing a consultant. For example, in the UK I would recommend: colourmanagement.net

[D Fosse]

There's really no need for a consultant, and there's no reason to settle for "close enough". Dave summed it all up perfectly in post #2 - just read and absorb. It's all there.

[D Fosse]

Oh - one thing: You need to get a specific CMYK profile from the printer - they will give you this if you ask.There's no such thing as generic "CMYK". If it's the wrong profile, it won't come out right.

[Norman Sanders]

I would like to add one comment to those already offered: unless you can control the viewing environment of the final printed piece (color temperature, color rendering index, surround, etc.), “exact” is an impossible goal. In the real world, the variation can break your heart.

[Derek Cross]

That's for sure!

[D Fosse]

Not sure I entirely agree with that - at least not in this case, where both end up as printed pieces seen together. What's needed here is a colorimetric match through two separate printing processes. That's perfectly achievable using color managed printing, as long as no color is out of gamut in one.

Admittedly, there is one wild card, which is what's known as metamerism failure. Certain inks may be perceived differently according to the spectral distribution of the light source. It will be most pronounced with spectrally irregular light sources like LED or fluorescent, but you can sometimes also see this with inkjet prints viewed in daylight vs. incandescent. A neutral color in one may take on a color cast in the other.

But even in monitor vs print you can get a surprisingly good match. The trick is to calibrate the monitor to match paper white and max ink. This should take into consideration the print viewing light. Few people do this, and that's why we have this myth that Screen And Print Will Never Match. It's not true.

[Theresa J]

peachytoo​ This thread is filled with some excellent information. Hopefully you have read through all of it. I’m guessing however that your head is probably spinning now. I’m going to try and explain WHY color management is necessary, in easy to understand terms.

Start with any CMYK mix of inks to print a color. For example 5c, 100m, 85y, 0k will produce a nice red color, but the actual hue of the red will differ everywhere you print it. The quality of inks differ depending on the press. Also, the qualities of each substrate differ. The specific way the ink interacts with each substrate determines the color. There are infinite possibilities of ink plus substrate combinations, even if you are only printing on fabrics.

RGB color mixes refer to the way colors are created via light, on your monitor for example. Color calibrating your own monitor is highly recommended for your own workflow, but you have no control over how someone else will view the colors on their own monitor.

Color Management is challenging. The first step is understanding why it is necessary. I hope this helps a little.

Theresa

[NB, colourmanagement]

Hi Peachytoo

you write:

I am trying to develop matching items, some in fabrics which printer requires RGB color codes, and others in paper, which printer requires CMYK color codes.  The key to this project is, I need them to match in color, or at least be a very, very, close match since they are intended to go together in a set.  I use Gimp for fabric and Photoshop for paper (conversion to pdf required).  I know how to change between codes and such, but I find every source I have looked at, none agree which number codes are a match.  Is one program, conversion chart, or any other source, the most accurate match in converting the codes that will result in matching finished products?  I have spent so much time and money trying to resolve this matching issue I am about ready to just give up.  Please help if you can.

davescm has given some useful basics, I’ll expand a little. Please take the time as it’s somewhat complex.

First you need to be sure that your computers display is set top to show the colours accurately, we call this calibration and profiling. It I uses a sensor on the screen to produce a table that, after calibration, defines the screen’s “ICC profile”

1: you mention RGB and CMYK colour “codes”.

Generally, I'll just call them RGB and CMYK colour numbers below.

The first very important point is to be aware that there are no generic RGB or CMYK values, that’s why you can’t find tables to switch accurately online.

To understand the visual colour that's represented by RGB and CMYK numbers, you need to know to which colourspace the numbers relate. The colourspace is defined in an ICC profile.

More here: about icc colour profiles, icc profiles explained | colourmanagement.net

Example,

showing the difference between two RGB colourspaces defining the SAME COLOUR:

sRGB                          R=240, G=60, B=35

is the same colour as

Adobe RGB(1998)       R=208, G=634, B=41

You can see that those numbers differ significantly, so if giving RGB numerical values you also have top give the relevant colourspace. In Photoshop this is called the “document profile”.

CMYK is similar, each CMYK colourspace (or profile) defines a particular printing process where inks and paper capability etc. may change -

Example:

if we convert the sRGB numbers above (240, 60, 35) to ICC profiles defining different print processes we get different ink recipes

FOR THE SAME COLOUR

convert to:

A: US Web Coated CMYK                          we get 0, 91,99,0

B: Coated FOGRA39 (ISO 12647-2:2004)   we get 0.86,86,0

If we send CMYK “A” to a (well set up) press running to US Web Coated CMYK we should se a very similar colour on paper* to when we send CMYK “B” to a press running to Coated FOGRA39 (ISO 12647-2:2004).

*of course colour match between process is dependent on gamut and tonal capability of the processes, colour management can’t make newsprint match Vogue [but it COULD make Vogue match newsprint]!

So you can see that there’s a bit to this, in terms of how RGB and CMYK are defined.

A solution:

I would consider sending a known colour chart [a profiling chart] to the RGB fabric process (with any colour management disabled) and after production, measure the end result using a spectrophotometer to define the appearance - so that we now know the RGB colourspace of that process - we now have a "fabric RGB" ICC profile.

[it’s that same process that makes any printer profile]

Now we have what we call an RGB “output” profile to define the production process.

It can be used for both softproofing (on screen) and as a destination for conversion of files before sending for production.

From now on:

If you’re working in Adobe RGB you can make the RGB file for the fabric process using that RGB profile.

(Convert to profile : Adobe RGB to “fabric RGB”).

When you send to print (press, inkjet), you convert to a relevant CMYK or RGB profile which defines that process.

IF the fabric process has a limited gamut (colour and tonal range) we can set both the screen and the other print process to simulate that gamut, so the colour range is restricted to match. That’s how softproofing or a printed proof works.

Of course, colourmanagement requires consistency, the monitors screen requires periodical recalibration, the printing processes must be well controlled, same for however the fabric is made.

I hope this helps

if so, please do mark my reply as "helpful" and if you're OK now, please mark it as "correct" below, so others who have similar issues can see the solution

thanks

neil barstow, colourmanagement

[Derek Cross]

Thanks Neil

I'm sure the OP, and others, will find this authoritative and detailed reply interesting and helpful.

If the OP could tag the question with the term "Color Management" it would help others find this thread in the future.

Derek

[davescm]

Hi Derek - I've tagged the question as you suggested

Dave

[NB, colourmanagement]

Hi davescm,

can we tag it "colour management" [note spelling] as well so it shows up for no US English speakers too when searching?

thanks

neil barstow, colourmanagement

[davescm]

Hi Neil

Done

Dave

[Derek Cross]

I've also posted a link to this thread in the InDesign forum, as I think many InDesign users will be interested in this subject.

[c.pfaffenbichler]

Please elaborate.