I really like the sandy hills. A couple of NES errors, though.
https://i.imgur.com/WRCd8JQ.gif
Red = more than 4 colors in a 16x16 pixel region (least common color in region marked)
Blue = more than 4 colors in an 8x8 region. (least common color in region marked)
Green = colors outside the available 4 color palettes.
Probably not worth changing (and really, it's totally possible with a few sprite overlays), I'm just cleaning up this error checking thing for release and this made a good test case.
This sounds like an after the fact excuse, but I'm actually aware and made those areas as a sprite overlay on a separate layer (which I hope wouldn't for some reason be impossible on the NES). I made sure to otherwise check that it's all valid using NES Screen Tool. Sorry for not specifying. Thank you for taking the time to put the picture under such scrutiny and I'm glad it made a good test subject : )