Coloring Paint Swatches, using a CIE Lab recipe?

Question

Disclaimer: I confess I don't know everything in Photoshop 🙂

I'm trying to prepare some demonstration for my students.

I would like to show them how to "re-color" an existing paint swatch. Here's the perfect application. Suppose I work fora paint company and asked to come up with new swatches for some new paint color? Naturally, I start off an existing collection of swatches such as this one :

And then what? My first "naive" approach was to make a copy of the white dab and try to change its color by using a Solid Fill adjustment layer in Color mode, filling the layer with my new LAB paint specification, such as L45 a-35 b+50. Some kind of green. But it did not work? Delete the Fill Adjustment layer.

Then I tried a Hue/Saturation Adjustment layer, first simply shifting the HSL sliders around and then, checkin the "Colorize" option, Not great.

At that point, I tried to "reason" the protuation (a combination of "Problem" and "Situation") and tought, perhaps, if I start on a "middle gray" dab, I'd have better luck? So I deleted my white dab and copied instead the Green dab, which has more "shape" to begin with.

Redid the exercise and thought I had "better luck".

Is there any "better" approach to this typical kind of "recoloring" problem?

D Fosse · Answer

If you look closer at the those samples, you'll see why that won't work.

Every color has a brightness. Yellow is inherently bright, purple is inherently dark. The whole tone curve is completely different. You cannot just replace one with the other, then you get a very pale lilac and a muddy olive brown.

In image editing, the color component has to be strictly separate from the luminance component. If they weren't separate, painting in Color blend mode would just be Normal blend mode, just a normal paintbrush.

White and black are not colors. They are the absence of color. They are pure luminance.

The confusion happens because in everyday speech, we use the term "color" in a very imprecise and generic way, to mean the combination of color and luminance.

Changing color of an object is a very difficult and tricky operation, requiring a lot of experience. You need to remap the whole tone response curve. That has nothing to do with Photoshop, it's just how we perceive different wavelengths. The eye is much more sensitive to yellow/green than to purple/blue.

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