Adding fuel to the fire means that you make a bad situation, particularly a fight, argument or something like that, worse. As for chasing rainbows, I'm not sure. Maybe it means pursuing something intangible or unreachable, like having a dream that you can't possibly fulfill.

I know those Chinese idioms."Adding fuel to the fire" mean making somebody more angrier or making something even more worse. "Chasing rainbows"mean a man who have a beyond himself aim and he can never make it.