Nick, here are 3.
Cyan - 2,4 dichloro napthol
Magenta - p-nitrophenyl acetonitrile
Yellow - acetyl 2,5 dichloroanilide
Dissolve any one at 1 g / 100 ml of solution and add to 1 liter of C41 developer. This should give a color image. In fact, if used on separation positives, and sandwiched together, you can make an ersatz Kodachrome.
The magenta and yellow in combination will make red, but the hue will vary depending on the ratio of the two couplers, as they don't react at the same rate.
These are not the best, they are most readily available and least expensive. The Kodachrome couplers are sold by Kodak, but cost an arm and a leg plus a few other body parts that come to mind.
PE