Wasn't sure where to post this.  Moderators, feel free to move.

I needed to output some generated colours to the user in all the usual formats (HSV, RGB, CMYK, Hex), and ran into the problem that there isn't a colorMode(CMYK) option.  I found this post from the alpha board: http://processing.org/discourse/yabb/YaBB.cgi?board=Tools;action=display;num=1082055374, but it seems to have been abandoned.  So, I used amoeba's pseudo-code from there to make a simple CMYK_Colour class (yes, Canadian spelling )  

The conversion is crude, but works for my purposes.  For printing to CMYK, use SimplePostScript: http://processing.org/hacks/hacks:ps.  Currently, this expects color objects to be in RGB mode.  However, it would be easy to adapt this to convert from HSV (HSB) colour objects.

Code:

class CMYK_Colour {

//fields
float cyan, magenta, yellow, black;

//constructor - requires colorMode(RGB,255) is set
CMYK_Colour(color c) {
//convert to CMY
cyan = 1 - (red(c) / 255);
magenta = 1 - (green(c) / 255);
yellow = 1 - (blue(c) / 255);
//convert to CMYK
black = 1; 
if (cyan < black) { black = cyan; }
if (magenta < black) { black = magenta; }
if (yellow < black) { black = yellow; }
cyan = ( cyan - black ) / ( 1 - black );
magenta = ( magenta - black ) / ( 1 - black );
yellow = ( yellow - black ) / ( 1 - black ); 
//convert to value between 0 and 100
cyan = cyan * 100;
magenta = magenta * 100;
yellow = yellow * 100;
black = black * 100;
}  

}       
 

Usage:

Code:

//set to colorMode(RGB) - this is the default
colorMode(RGB, 255);
//create colour object (pacific blue)
color swatch = color(0,128,255);
//create CMYK colour object
CMYK_Colour cmyk_swatch = new CMYK_Colour(swatch);
//output CMYK values (these are floats)
println("CYAN: " + cmyk_swatch.cyan);
println("MAGENTA: " + cmyk_swatch.magenta);
println("YELLOW: " + cmyk_swatch.yellow);
println("BLACK: " + cmyk_swatch.black);
 

Hope someone else finds this useful.  If there are serious flaws, or minor improvements, please post back here!