The flutist

I love the limited palette you used, though I think that for the shadows, it could use some other subtle hues. Like for the piper's clothes, try using some purples and greens on the grays, along with browns, and vary the hues on the mice too. I do like the white-ish fur lines, as they really make the mice stand out. Try making the shading with some different colours too.

As for the piper's fur, give him (or her) some colour variations too. Maybe add some blues and browns for the shading. For the stalactites and stalagmites, add some more subtle cyan and blue, and maybe a little bit of purple and brown. Experiment with it.

Either way, you did a great job. Nice composition, and very creative!