Entering CMYK colors into Prism Mac

 When you choose colors in Prism Mac, you use the standard Mac color dialog. The buttons across the top offer five ways to pick a color. If you don't see the buttons, they are easy to bring back.  In addition to those five methods, you can also use the magnifying glass to choose any color shown on your displays. Troubleshooting the color picker.

The second button lets you choose a color by entering values. The drop down offers four different color schemes. The CMYK choices are shown below.

Entering colors by choosing values for C, M, Y and K presents a problem. Prism stores colors internally in RGB (red, green, blue) format. Every color can be encoded by three values (R, G and B). That means that every color can be encoded by many combinations of C, M, Y and K.

Whenever you move one of those four sliders to choose a different color,  OSX converts the color to RGB (taking into account the settings in the color profile set in the Display system preferences), and then converts it back into the preferred CMYK combination. If you look at the numbers and sliders, it feels out of control. After you move one slider to a new position, all four sliders automatically move. But if you focus on the color swatch on top of the dialog, you can see that it works fine. All OSX is doing is encoding your selected color with a different -- but equivalent - set of CMYK values. 

This problem does not occur with Prism Windows, because the Windows Choose Color dialog only let you choose by entering RGB (or HSB) values, and does not let you enter CMYK values.