While each is more or less monochrome, they're in spectral order, ranging from red through purple, blue, green, yellow, orange, and back to red again. In fact – get ready, I'm about to blow your mind here – in fact, I say, a certain amount of transitional coloration on the left-hand side of each image leads me to conjecture that they're all actually part of a crazy super-long Edgar Rice Burroughs rainbow mural.
Here I've just stuck the covers together so that you can get a general idea of what I'm talking about. They don't quite line up at the edges, and skip space between successive covers here and there. Maybe they're from several panels rather than a single one. In some instances at least I seem to see a single horizon extending from one cover to the next. Searching around the Internet yields images that corroborate the idea without quite confirming it. Here's the artist's website, which makes me think, eh, maybe not, but it's still cool to think.
Finally, here's one wrap-around cover painting by Michael Whelan (copyright 1979). He's a well known fantasy artist – he also did a very creepy wrap-around painting for a volume of H. P. Lovecraft stories that I own – and you can see much better images of his various Burroughs illustrations by performing a judicious Google Image search. The Thuvia painting is my favorite, because, with its dusky, dusty golds and blues, it most closely resembles how I imagine Barsoom.