Yellow should be referred to as "brighter yellow" or "darker yellow", yellower does sound bad, you are right.

Well-off usually means "rich"

better-off is probably what you want to use, It would help to have a bit more context though. "Susan was better-off without him" would be a way to use that word. Good, better, best are some other options you might want to try.

More well-off is not used.

good looking and better looking can both be used. "She was good looking." "Susan was better looking than Katie." "Susan was the best looking girl in the room".