Paint color suggestions for redheads?

I think it depends on what technique youre going to use to paint, because that also affects the final shades.

I usually drybrush hair, so I'd do a base of dark reddish brown, drybrush some red on it til im happy with the coverage, then drybrush a few highlights with a reddish orange. It looks okay for comic book customs.

If you want more natural shades, use darker reds.

If you go with simply painting the hair a solid color and shading the groves with a wash, that's going to change your final color too.

Get some normal red and simply mix brown or black into it as needed, experiment.

One thing I'd been thinking of was, after priming, doing a yellow wash, then an orange/brown mix wash, then a red wash over that. (Cosmetologist training here- real human hair has layers of pigment made of different-sized globules of melanin, which is why, when you bleach your hair blonde, you'll find orange bands in it; pheolemanin, which creates red and gold pigment in human hair, is the hardest to dislodge.) But then I kind of got anxious that that would look terrible and not turn out at all how I wanted, and thought I'd better ask someone who knows what they're doing. /o\

I think adding those multiple layers may look good if you were rooting hair on a doll, im not sure the effect you want would translate well into washes, which like to puddle unpredictably.

Give it a try anyway, might look cool