I think the environment is a bit too busy/noisy, it kind of has that old point and click style which everything looks "dirty". It might be what you're going for but I have a few suggestions.

tiles: clearly you're working with the whole tile there, you might want to give yourself a bit more breathing space to the platform for the character looks more interesting visually.

atmospheric perspective: you were almost there on the trees but you need to work on your mountain range, give it a color to blend a little better with the sky.

shading: the clouds are ok(you can add an extra color for shading tho) but the mountains could have a shape better defined. this also goes together with the trees, the ones in the back could be shaded with more broad shapes.

Two new scenes, Toriel's house and Sans encounter. I'd say I'm satisfied on how tSans scene turned out ,but I'm struggling to finish Toriel's house because of the fireplace. The "bulk" part is pretty much done, but those details.. specially getting a good shiny-golden-metal-texture on the side supports. I'm looking for a pixel art metal reference, I'm 100% sure I had somewhere and it's nowhere to be found now, if someone has an image or some tips it'd help me a lot, thanks

Didn't expect to be that hard to draw Papyrus, I'm sure I'll wake up tomorrow and find a bunch of mistakes.


Thanks Curly/Pizza Tom! 

tbh I was going for a washed out look putting the colors close together but now, I see your edit and I'm reconsidering this approach to something more cheerful haha. It worked well on Frisk because he/she has more colors than Sans..

(yeah the sans thing, when I get to edit the original ones I'll take his eye off, forgot to do it before actually.)

I'd suggest doing some work on the texture part, Here is a basic tutorial about it, nothing really in depth but I believe you'll see a lot of improvement if you follow it.
Unfortunately I don't have something similar for hay but you should do a little research, studying textures is really important to convey the kind of surface that you want.

source (i believe)

colors, so you said something spooky? desaturated colors, purples, greens.. grimy colors might be a good start. This might be a little hard at first but using horror games as reference might make it a little easier. 

I'd also suggest looking for books on color from a design perspective since they tend to have a less physical approach to colors and more of a cognitive/psychologic. I'm not sure this is the right one (due to different versions) but if it is, "color design workbook" would be a good example.