Photoshop colour changes CMYK value when picking

Question

I can make an RGB document, choose a black of say C, M & Y all at 20% K at 100% fill this colour in. Now when running picker on it, Photoshop reverts it to its generic black! I never had this problem with past versions. What is happening now? Any help would be great as I prefer to make backgrounds in RGB mode as colour seems to be better when printing. I used to be able to control the black, but now I'm stumped...

Of course it’s an RGB file just like printing a photograph which is RGB to CMYK, it has to convert. There is no point having the CMYK picker if you can't mimic that colour. In the past you could choose your black paint, and even though in reality it was RGB (because it’s in an RGB file), it would still return to you the correct CMYK value that you chose and mimicked in the first place. PS no longer seems to do that. You choose your false CMYK colour to paint into the document and then when you check it, the values are, well, different. Totally pointless having any colour picker as now you can’t control it. That’s what I'm referring to. The colour you mimic should still return the same values that it mimicked in the first place.

Answer 1

Do you know those cubic watermelons?

You can assign a cubic shape to the watermelon... but when you plant a seed, guess what shape you will get.

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An RGB file is an RGB file. The information it saves is RGB. When you assign a CMYK value, it is converted to RGB.

The "Generic" black is not a generic black. It is the black that will be generated if you change the mode to CMYK using a specific color profile.

On a PSD file format, you can save different information, for example, some Pantone Channel, but a normal RGB file will save only RGB information.

Answer 2

Here is a short explanation:

First of all you must realize that, unlike in RGB, there can be many different combinations of CMYK values which yields the same perceived color on print. Theoretically it should be sufficient to print with just CMY, but the K (black) has been introduced to keep the total amount of ink down and to make it easier to print black text and strokes. This addition of black makes CMYK more complicated to understand than RGB.

What you experience when selecting CMYK colors in an RGB document is to be expected. What happens is this:

• You select a CMYK color.
• Photoshop asks its Working CMYK profile: "How would this CMYK color look if it was printed by a printer which follows your standard?" and converts the color to the absolute color mode Lab.
• Photoshop then asks the RGB color profile of the document: "How would this absolute color be defined if it were to be shown on a screen which follows your standard?" and the color is converted to RGB.

You use the RGB color in your document. Photoshop doesn't remember the initial CMYK values - only the resulting RGB values. When you later measure that same color the opposite happens:

• You select an RGB color.
• Photoshop asks the RGB profile of the document: "How would this color look if it was displayed on a screen which follows your standard" and converts the color to the absolute color mode Lab.
• Photoshop then asks the Working CMYK profile: "How would this absolute color be defined if it were to be printed by a printer which follows your standard?" and the color is converted to CMYK.

The resulting CMYK color will be defined by a wide range of choices made by the people who made the color profile. Too complicated for me to explain here. The values will probably differ from the values you chose in the first place.