Tabletop Miniatures

How do I decide the color scheme of priming? It's a hard question to answer because many different approaches can work. I believe the technique is called underpainting, especially in the fine arts world. For example, many clasical painters will paint skin green first as an underpaint before making them more skin tones.

One heuristic you could use would be to think about the light color coming from the sky and reflecting from ground. For example, a warm sunny day over a forest floor would be yellowish from above and green from below.

But I think the two color priming can lead to interesting results regardless of what colors you choose. Though I might try to make the under color darker in value that the over color.

For fire skinned orcs, if you're going for glowing from within I would base white followed by layers/drybrushing of yellow, orange, red, then brown.

If you want red skin I think your idea could be super interesting. Just make sure you mix some of those colors into your reds as you layer up, as well as letting some of those colors show through in the deepest shadows.

Keep on practicing and exploring. You're asking great questions!