Well if you have 7 layers of flat colour in photoshop, you can set the layer mode to dissolve and opacity to 14.29% incrementally on each layer iirc... for example if just red, green and blue.... top layer 33%, middle layer 66%, bottom layer 100%, and that's if you're supposed to have (or want to have) a perfectly even amount of each colour.
You'd have to relate this pretty well to alteernative analogue processes to have this kind of discussion on here.
If youre going for specific colour values as opposed to full saturation (max value of one channel such as R, while 0 for all the others), then you need a calibrated monitor and printer to make sure your screen prints come out as they look.