说某人在chasing rainbows，就是说他在追求不现实的幻想，有点“夸父追日”的意思。

I know you want to become an actor but I think you should stop chasing rainbows and get an office job. Becoming a successful actor is really hard!
Bob had hundreds of plans and wild ideas about how he could become rich, but none of them ever succeeded. He was always chasing rainbows.
值得注意的是，a wild goose chase 字面上的意思是“追逐野鹅”，形容那种花很多的时间和精力去徒劳的追求某件事情，而这样的追求不是没有方向就是方向错误。

We were told there was an excellent Korean restaurant in town and spent two hours looking for it, but we couldn’t find it. We later found out it has closed down. What a wild goose chase!