RGB & CMYK

In defense of reality (and lithographers the world over): 

While I appreciate the fact that such words as “exactly” and “perfect” are part of our rich vocabulary, I have learned to take them with a grain  of aspirin. I suggest you do the same when considering color printing. 

I offer this advice based on my many years printing for museums, major corporations and some of the most demanding art directors and photographers on the planet. 

Appreciate the fact that if you cannot always control the light source under which the color is viewed, there is only one way to get a perfect match every time under those varying conditions.

Lower your standards.  

If you don’t already know about the limitations of reproducing RGB colors in CMYK — especially blue — these links might help explain things.

Color Distribution, Printable and Otherwise by Dan Margulis

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…no secret: saturated blues are difficult to impossible in all print conditions… 

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Why Does Blue Turn Purple? (in CMYK printing)

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Conventional CMYK isn't just slightly worse than RGB at producing blues--it's drastically, horribly worse. There isn't any separation method that can cater to every possibility. The following are the basic options… 

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Regarding this requirement…

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@Christina McG wrote:

I don't want to change my branding  - I need my material to look the same. 

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Because the problem of reproducing blue in CMYK print has always existed, brands historically handle that in ways like these:

• Establishing a brand color that is known to be reproducible using CMYK inks. 
• If a brand color cannot be reproduced in CMYK, find an acceptably similar reproducible CMYK blue to be the brand color when used in print, even though it may not be the same as the non-CMYK color. 
• Keep the brand color that cannot be reproduced in CMYK but make sure there is a spot color ink that can, then always require that color to be printed with that spot ink, because a spot color can be outside CMYK. The disadvantage is that you will always be paying for one more printing plate. 

For example, a company might say “Our brand color is PANTONE 2726 CP from the Pantone Color Bridge system.” They picked that because that blue is reproducible in CMYK, using the values shown in the Photoshop color picker. Blues are consistent on screen and on press.

If a company says “Our brand color is PANTONE 2727 C” without thinking of CMYK limitations, and the color values are outside the intended CMYK process (as set in Color Settings), the Photoshop color picker will show them that the blue color values are out of gamut for printing — it won’t look the same. If the brand color cannot be changed, an option is to pay for an additional a spot color plate for PANTONE 2727 C ink, or use the closest printable CMYK blue and live with it.

Let's be clear on one thing first: it does not necessarily have to be CMYK just because you're printing it.

CMYK is for commercial offset printing in large volumes. In that case you absolutely must use the CMYK profile appropriate for the specific printing process. You need to get this profile from the printer.

For inkjet printing, your files should remain RGB. Do not convert to CMYK, it will give an inferior result. Most low volume jobs are printed on inkjet printers.

So - are you still sure you need CMYK?

Please set the Status Bar to »Document Profile« and post meaningful screenshots. 
Which RGB? Which CMYK? 

There are some colors in the RGB color space that simply cannont be created with CMYK; bright blues are among those. If a particular color is critical, you will have to pay for a spot ink. Otherwise, post a sample of your material here, and you may get some suggestions for getting closer to what you want to acheive.