Converting between RGB, CMYK and #nnnnnn

Posted on 2009-04-21
Last Modified: 2012-05-06
I am a web developer, I occasionally do a little graphics but mostly programming. We hired a graphics company to change our branding (new logo, company colors ect).  

Yesterday I was provided the color codes in all three formats (RGB,CMYK and #nnnnnn) for the new colors. I think they got mixed up when recording the color codes.

Photoshop is coming up with different conversions between the formats than what the graphics company provided.

For instance if I enter an RGB provided by the graphics company in Photoshop, Photoshop will display a different CMYK and #nnnnnn than that provided by the graphics company.

Is there any reason why this would be intentional?
Question by:dmoss123
    LVL 4

    Accepted Solution

    Your version of Photoshop, and their version of Photoshop (which they probably used to get the conversions) might be in a different color space mode.  You may want to double check the color space mode of your document, and Photoshop's color settings (i.e. Edit > Color Settings), then you should probably make sure it matches up with the color settings of the individual who produced the logo for you.  It also depends on the bit depth capable of your machine.  If your video card cannot match the bit depth does not match that of the bit-depth on the design'er machines, then you will most certainly have "missing" colors that Photoshop simply cannot find.

    LVL 16

    Assisted Solution

    Agreed. You can ask them to give you a screen shot of their color settings and compare to yours - most likely they are using a different ICC profile than you are - which they should be able to provide you with.
    LVL 18

    Assisted Solution

    You may want take a look on color schemas math and conversion algorithms.
    The conversion from one system to the other then back to first one can result in different values from the original ones, due the nature of subtractive and additive processes.
    See Example 1, the conversion from CYMK to RGB, then from the found RGB values back to CMYK.
    The final CMYK values are different from the starting values. Why? Because in the original color definition, it is defined just by its CMY components. The conversion from the RGB to CMYK considers the CMY sum to a grey value (0.3), and eliminates such grey component from the CMY components.
    In example 2 we see what happens. We do the same conversion: from CMYK to  RGB then back to CMYK. Which color is it? It is BLACK. The original black was obtained by the CMY at 100% each, thus resulting in RGB = 0. When we convert such color, black, to CMYK, the conversion process combines the CMY components into the K, thus resulkting 100% K, resting nothing to CMY, reduced to zero.
    To avoid the differences on conversions, we can start with the RGB definition then converting it to CMYK.
    Of course, this explanation doesn't conflict with the ICC profile possible cause. If the main objective is for printing, then Pantone definition is an appropriated choice. If the target is internet, then RGB websafe is the best reference.

    Example 1
     C     Y     M     K              R    G    B           HEX
    0.8   0.3   0.4   0.0    --->    051  178  153    =    33B299
    0.71  0.0   0.14  0.3    <---    051  178  153    =    33B299
    Example 2
     C     Y     M     K              R    G    B           HEX
    1.0   1.0   1.0   0.0    --->     0    0    0     =    000000
    0.0   0.0   0.0   1.0    <---     0    0    0     =    000000

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    LVL 16

    Assisted Solution

    Jose, where you been man? Long time!
    Very good points - I never delved that deeply into it, but just the point about the additive and subtractive schemes makes sense to me.
    LVL 18

    Assisted Solution

    Hi, BongSoo. Yes, I am walking in algorithm and 3d tas. Thanks for the warm "hello". I'm glad to see you around too.
    In continuing the answer to dmoss123, on should note that slight differences are allowed, due RGB formats (32 bits w/Alpha channel, 24 bits, 16 bits, 16 bits palletezed), or rounding schemas. In fact, it is surprising that a RGB value turns into wrong CMYK, when originated at a specialized house... Probably they can explain, for example, if such values are differente but, in some way, equivalent, as CMYK (100,100,100,0) are note exactly a BLACK nor CMYK (0,0,0,100) isn't a perfect BLACK (In some cases I use to make CMYK (100,100,100,100) for "strong" BLACK in print maerial).

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