It is best to give red envelopes on the same day for good luck. Under normal circumstances, when you meet the bride and groom, you can shake hands first, then hand in a red envelope and say a few words of blessing, which is decent and generous.
If the red envelope you prepared can hold greeting cards, you'd better give it with money. If packaged separately, the red envelope must be red, and it is best to choose red for the packaging of greeting cards. Of course, there is no such one, and other colors will do (try to avoid black).