Did some reading, and tried remaking those tiles. I still need to add full borders to them, but is this the right track? Should I try to have all of my tiles share the same palette, or is it ok to have different color palettes between, say, ice and cave tiles? I guess this is all stylistic but I'm still not experienced enough to have opinions on my own style...
EDIT: put together, with some snow:
Hrm, I chose background colors pretty quickly based on contrast w/ the foreground, and I think that works, but they don't really fit the scene. Does it work to have similar bg / fg hue and just desaturate + brighten/darken a lot?
EDIT II: Yeah, looks better like this I think: