Your color variations are very good. If you are going for a "stacked" look like some mountain ranges look like, these could pass.
If you would like the blend in these stacks of foam to look like one big mountain, there are some tricks your could try.
1st - You have some nice random cuts. Try do more of that, but then fit them together like a broken puzzle.
2nd - Get a soldering iron and make deep drags through the entire structure. Very random.
3rd - Use a dull #11 blade and start slashing. This part if fun... especially if you are mad.
3rd - Take a wire brush to the whole thing. Don't worry if big chunks fall off. You can place them elsewhere as boulders if you wish.
I'd rather be Lucky than Good.