I jumped briefly into CMYK to grab copies of the C, M, Y and K channels, before returning to RGB.
I created 4 'solid color' adjustment layers, one each for CMY and K.
I masked each layer with the negative of its alpha channel *
I multiplied them all together.
The layered .psd is here, if you'd like a closer look.
* To mask a layer with an alpha: go to the alpha channel you want. Click on it while pressing the 'control' key on a PC. That should load the alpha channel as a selection. Now with the selection still loaded, ask for a layer mask on the layer you wish to mask. It will materialize with the selection already punched into it. Finally, hit control-i on a PC to invert the layer mask, which is a necessary step in following along above.
So 'subtractive color' might be better called 'multiplicative color.'
If the color match in this example isn't perfect, I plead RGB <-> CMYK gamut and Photoshop color management and colorspace issues.
UPDATE 6/2/2009
According to Getting It Right in Print by Mark Gattner, the K in CMYK was not chosen because it disambiguates from the B in Blue. The K in CMYK stands for 'key,' as in 'key plate,' the plate (often with the text) to which the other colors are registered.
UPDATE 6/15/2009
An in depth look at the linear equations and non-linear lookup tables that more closely approximate the RGB to CMY conversion can be found in Digital Color Halftoning by Henry R. Kang
Finally...
... for fun, I changed the colors of the definitions of the 'inks' - magenta is now lilac, for example - and I threw a gamma curve onto the distribution of the cyan 'ink.'
Okay, I'm on a roll, now...
Now that I have the CMYK channels as layers, I can address them separately. I can move one, for a mistake-in-printing look. And I can apply filters to each one separately.
As a goof, I applied Filters > Stylize > Wind twice to each of the CMYK layers - once from the left, and once from the right.
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