In creating a Martian-themed gaming table , resplendant with multiple craters, I faced a painting issue: what colour should the interiors of craters be?
(i) the same colour as the exterior?
(ii) a different tone to the exterior?
(iii) a different colour to the exterior?
I can see arguments in favour of all three, depending on the geology of the planet in questions. But eventually, I decided that it didn't matter too much: this is science fiction and therefore we're at liberty to come up with something ourselves. That said, I didn't want luminous green crater interiors ... just something that looked plausible, if not "right" (whatever that means).
Accordingly, I began to paint the interiors of the craters in a slightly darker tone to the exterior, with a touch more brown paint added. In the image below, I've tweaked the colour contrast settings to bring out the change in colour more than is apparent in real life (just for effect). The interior does indeed look more brown than the exterior. But, I don't think it looks odd: I'd contend it looks quite a plausible colour for the terrain to be. There were a couple of layers of watered down black paint applied to this surface to give it a recessed wash, but the effect is there. The final step will be to highlight the rim of the crater in red (or perhaps a red/orange blend).