I think the sea should be taken more to the ultramarine side of blue - which is what you meant by a sterner feeling isn't it
I agree on leaving the forest not coloured, but I also notice that you have used a very cold almost silver grey to do the skull. Perhaps a colour more in line with the parchment itself? Bone is astonishingly warm in colour unless its been bleached by the sun, and even then its still a warm white rather than a cold white.
The mountains might look more impressively 'mountainy' if they weren't so minutely fractured, but don't take it too far the other way
Looking forward to seeing the next WIP!