What does 别把这天的彩虹, 放在明天的天空 mean?
别把这天的彩虹放在明天的天空

Don't take today's rainbow and put it on tomorrow's sky.

It should mean something like:

If you are happy now, it doesn't mean you will still be happy in the future.
If things are good now, it doesn't mean they will still be in the future.

Or simply:
Things change.