A little bit about color separation filters, for fun:
The eye has 3 "color sensation nerves", red, green and blue. When all are present in equal proportion we see white. Combinations of the 3 produce all the colours we can see.
So the basis of colour reproduction in photography is to separate these three sensation colors thru the use of filters. A deep red filter (25, 29..) passes more or less only red to the negative and thus you have your red separation. A deep blue filter (47B) passes only blue to produce the blue separation negative, and the green (58) filter passes only green to produce the green sep. The modern films we use do this at once, in an integral pack, though instead of using 3 filters, the sensitivities of the 3 layers are made so that each one only records their respective color (though there is a yellow filter after the front element, but that's another story altogether...)
Each negative is then a record of only that particular color sensation. If one where to turn these negatives into positives and project them
thru their respective filters onto a screen where all three overlapped, you would get a full color image synthesized by the additive method. This principle is how screen-plates work (autochromes, dufaycolor...) and your TV. No modern photo system uses this, unless you consider digital display (which I don't!
).
Subtractive color is slightly harder to wrap your head around, but the idea is to subtract color from white light, as opposed to adding colored light to darkness. All modern films, E6 & C41 and all modern papers, Ilfochrome & RA4, use subtracive synthesis. Assuming we have separation positives, then we must turn the shadow regions into the
complementary color of whatever filter that separation was shot thru. So your red filter positive needs to have cyan shadows since cyan is the opposite of red and since the shadows are dark, that means they have no or little red in them. The blue separation needs to have yellow shadows to subtract blue where we don't want it, and the green separation needs to have magenta shadows. Combined, the 3 colors will block all light and produce black, where different amounts of the 3 overlap we get all of the natural colors. Cyan & magenta will subtract red & green from white light to produce blue; cyan & yellow will subtract red & blue from white light to produce green, yellow & magenta will overlap to subtract blue & green light to produce red. The complementary colors to the additive primaries are, logically enough, referred to as the
subtractive primaries.
And that's where babies come from!