Some NES-Style Level Art

[hi19hi19]

Diabetes Land is extremely Super Mario-ish. I actually like it the most in terms of a consistent feel.

The background of Nightmare Castle is great, but the palette choice for the foreground is pretty lame. The flat grey wall and the bright green grass really don't fit the mood.
For this kind of theme, I'd definitely take inspiration from the artwork for the old NES Castlevania game. Not really the most spectacular technical use of the NES's graphics, but I always thought they did a damn good job with the atmosphere.
Check this shit out:

You can see the foliage green is dark and a tinge bluish to go with the bluish highlight on the gravestone, and the green on the cloak in the middle is also a faded, earthy olive tone. Really sells the graveyard feel.