Doing CMYK is actually fairly difficult, everything is set up for RGB. I set up a sketch that gives the RGB value wherever you click. I wrote an explanation on why this works recently, I'll try to find it in a minute:

Scroll down to about the bottom of this topic:  http://forum.processing.org/topic/how-to-change-the-color-of-each-pixel-in-an-image#25080000001997385

Brightness: see the function of same name in the Reference, and look at colorMode() too (HSB).

could i plot each individual colour (or combination of colours) over a black background, and then take the brightness.
ie: combine red and blue, over a black background, therefore effective brightness would be magenta?

Brightness is different from color, it actually removes any color and produces a value (grayscale).

perhaps i missed out a crucial piece of information.

the data is to be used to plot an image onto paper.
The setup currently pulls out greyscale (brightness), and plots it resulting in a pretty good black and white likeness.
I have modified the setup so it can loop over as many times as needs be, through all the colours required.
being additive rather than subtractive colours (or is it the other way around) rather than RGB,
So the resultant image data needs to look a bit like this:



cycling once for each colour, and plotting full colour as 255, white as 0....

as they are difference cycles, can i change the "colour mode" for each cycle?
so plot the saturation on each cycle, but limit the hues availible?

In other words, you need CMYK. As indicated by the posts above, Processing doesn't support CMYK natively (only RGB and HSB), which means you either write your own RGB to CMYK conversion methods (for formula's see google) or you use a library such as Toxiclibs which you can use to easily extract CMYK values from a color, see the TColor class.