To put it shortly, use complementary colour combinations.
It just seems to work, complementary colour ramps just look better than monochromatic ones, more unique, and more interesting to look at I guess.
Unlike hue shifting, which goes from yellow, to orange, to red, etc.., complementary colours just go from one side of the colour wheel to the other, red to cyan, violet to green, and yellow to blue, for example.
Though, you would obviously need some colours to connect between them (hue shifting), otherwise it'd look like a total mess (that being said, it also depends on the contrast and saturation of the two complementary colours, so technically you could have purple and green next to each other if they're somewhat desaturated or aren't that far away with the hue).
Additionally, you could go a little away from the direct opposite of one colour, for example, rather than going with red and cyan, you could go with red and green; or just use the RYB colour wheel altogether if you're feeling artistic.
Also, here's a rough edit of your rock. Pretty shit, but it [sort of] demonstrates what I meant.