11 Replies Latest reply on Mar 9, 2013 6:56 AM by c.pfaffenbichler

    RGB to CMYK color transformation

    w345 Level 1

      I know RGB to CMYK color transformation may not exact. But how could it have so much difference?

       

      I have a RGB = (28,97,255) want to transform into CMYK.

      Which number should I use?

        • 1. Re: RGB to CMYK color transformation
          ssprengel Adobe Community Professional & MVP

          Color numbers are in terms of a color space.

           

          PS / Edit / Color Settings / Working Spaces:  RGB set what you want, CMYK set what you want

           

          PS / Window / Color or F6 to turn on the color picker if it’s not already, or do again, if you’ve just turned it off, click the Color tab

           

          Click the little arrow-4-bars menu icon at the right of where it says Color and Swatches in the color-picker and set RGB if not already

           

          Type in your three numbers at the end of the sliders

           

          Click the little arrow-4-bars menu icon again and set CMYK

           

          Read the converted numbers at the end of the sliders

           

          Set it back to RGB, again, and make sure the numbers are the same, otherwise something might have been lost in the translation

          • 2. Re: RGB to CMYK color transformation
            gator soup Level 4

            also, read up on out of gamut colors & Soft Proofing in Photoshop, this image may help

            click on the thumbnail and drag the popup Granger Chart onto the desktop

             

            open it in Photoshop (use the embedded profile)

             

            Granger_Chart_RGB.jpg

             

            go to: View> Proof Setup> Working CMYK (this is "soft proofing" in Photoshop)

             

            Command+Y (Mac) to toggel it on/off

             

            notice those pretty saturated colors in RGB mode don't translate well to CMYK mode (they are out of gamut)...

            • 3. Re: RGB to CMYK color transformation
              w345 Level 1

              I don't quite understand what you mean, since those cmmand are quite new to me.

               

              Presumably you are saying the visible difference is due to the color profile for different device I am using?

               

              The RGB (28,97,255) is equal to CMYK (81,63,0,0)

               

              But the resulting color is very different. Are you saying that is only because the profile I am choosing? If I send it to galley print

              there won't have any difference?

              • 4. Re: RGB to CMYK color transformation
                ssprengel Adobe Community Professional & MVP

                What are the RGB and CMYK workspaces you have set in Color Settings?

                 

                If I choose the North America General Purpose 2 defaults of sRGB and US Web Coated (SWOP) V2 then entering RGB of 28,97,255 converts to CMYK of 100,32,3,1 with a 100 being a sign that I have maxed out the cyan color which may or may not be good.

                 

                Furthermore, there is a little warning triangle-exclamation-point at the lower left of the sliders and hovering says “Warning: out of gamut for printing”

                 

                If I click on that warning triangle then convert back to RGB the numbers have changed to a much duller blue:  74,109,177.  This means that the bright blue in RGB cannot be printed using CMYK inks, only the duller color blue.

                 

                RGB colors are in terms of luminous phosphors of a CRT or light coming from LEDs in your monitor whereas CMYK colors can only ever be as bright as reflected light from white paper that is dimmer than the glowing of a computer monitor.

                 

                The colors from your print will be more like the duller CMYK colors not the bright RGB colors.

                 

                What is the overall process you are involved in where you’ve asked to convert one RGB color to another CMYK color?

                • 5. Re: RGB to CMYK color transformation
                  Willi Adelberger Most Valuable Participant

                  You should avoid any conversion from RGB to CMYK in Photoshop at all. Keep your images always in RGB. If they are printed, eg. in InDesign, place the images with their RGB Color Space in inDesign and let InDesign do the conversion job upon PDF export.

                  There exists never an equal value from any RGB-Space to any CMYK-Space, what you get is only a closest by value under specific conditions.

                  As long as you don't know theses conditions you should never make any conversions.

                   

                  It is important that the color is correct in the choosen color space. The color space is technicaly defined by the color mode (in your cas rgb) and the profile, and out of technical bounderies also by the environment where you see these colors.

                  • 6. Re: RGB to CMYK color transformation
                    c.pfaffenbichler Level 8

                    Your use of

                    The RGB (28,97,255) is equal to CMYK (81,63,0,0)

                    seems to indicate that you might assume that RGB and CMYK values in and of themselves have an absolute meaning.

                    But they don’t unless the Color Space is declared (usually in the form of an ICC profile).

                    They can indicate that a pixel is more red, darker, … than another pixel in an image but the RGB values themselves do not define an absolute color impression.

                    If no profile is embedded in a file Photoshop assumes its Working Space for the respective Color Mode.

                    • 7. Re: RGB to CMYK color transformation
                      w345 Level 1

                      I  am doing galley printing for book cover, see the picture attached.

                      I printed a previous version and want to modify a new version.

                      (Galley printing normally requires CMYK, although RGB also acceptable.)

                      But I found in the previous version I actually used RGB.

                      (The background color was picked by eyedropper from the photo on the left....might from the photo on the right..cannot remember)

                       

                      The color profile I used are all default setting.

                       

                      I know the conversion may not be exact. But in all my previous work they all converted well.

                      Of course I can choose a new color. Just surprised at what happened.

                      biofluiddynamics.jpg

                      • 8. Re: RGB to CMYK color transformation
                        OldBob1957 Level 4

                        The attached image may help. Note that the entire CMYK color space is only a small subset of the colors possible in RGB.

                         

                        gamut.png

                        • 9. Re: RGB to CMYK color transformation
                          c.pfaffenbichler Level 8

                          Which RGB space specifically is that supposed to represent and which CMYK space?

                          Because those naturally can vary considerably.

                           

                          Edit: I am afraid graphics like that could lead people to the faulty conclusion that there is one RGB and one CMYK space.

                          • 10. Re: RGB to CMYK color transformation
                            OldBob1957 Level 4

                            c.pfaffenbichler wrote:

                             

                             

                            Edit: I am afraid graphics like that could lead people to the faulty conclusion that there is one RGB and one CMYK space.

                            You know, that is a very good point. I had never considered that pitfall. I shall be more specific with my graphics from now on.

                             

                            In any case; the above is actually from a series merely introducing the concept of "gamut" for those who may not have heard / do not understands the term, and is meant only to introduce the reality that all color spaces are not equal.

                             

                            That said, I believe I shall consign that particular graphic to the trash can.

                             

                            Thanks.

                             

                            --OB

                            • 11. Re: RGB to CMYK color transformation
                              c.pfaffenbichler Level 8

                              I hope I didn’t come off as purely negative.

                              It’s just that Color Management seems to be a bit confusing for people who confront the issue for the first time and the clearer the concepts and differences are presented the better, I guess.