Hi there :D

first, I want to say that I am not a good pixel artist yet, so I am commenting hoping that I can get something to learn too.

I have opinion regarding the black outline, I always try to avoid total black outline if possible, maybe dark blue or dark purple would do the trick... I don't want to be rude, but if it's because your colorblindness, I don't know what else to say.

I'm really sorry if I came out as a rude person :(

Honestly my biggest complaint about this piece isn't the colours, but the values. The dark outline on the bank blends with the characters, I recommend using one of your midtones instead, or maybe erasing it entirely since there's already another outline within the black outline. The  shadows on the rat(?)'s fur are a bit faint, as are the shadows and marking on the money bag.

A related "issue" is the dirt in the foreground. The fact that it has a  dark outline and a light-ish outline outside of that makes it stand out more than it's probably meant to. Maybe get rid of the light outline? You could also make the dirt's main colour a little darker so that it reads more as a solid, safe-to-ignore shape.


Colour feedback:

Money bags are typically depicted as more tan than green, and it's the $ sign that's usually green (or black or golden). I'm not sure that a green bag will read well to players. If you want green money, how about making the money drop as bundles of paper cash instead of bags of coins?

The outline on the background hills is green, which is pretty strange since they're brown. I recommend replacing the outline with the shadow colour from the hills - it'll be a very faint outline that'll help make the hills look distant.

If you're going for a green/red contrast between the characters, consider making the red character's fur a little redder, not so grey. That'll emphasise the contrast and look a bit more exciting. If that's not what you're going for, the current colour is fine.