With the pipes, I'm trying not to make it look like a rehash of the SMB pipes, as were many other custom pipes I've seen. The SMB pipes look like they have a kind of sheen, which doesn't quite fit with the other tiles, which have minimal shine.
Nonetheless, I tried to shade it using a somewhat similar style to SMB:
As for Quint, maybe MM9/10-styled. That's what people say when their MM sprites break the "3-color rule". :D