Just started on CotC and idk what to do…

Very new and totally basic team comp right now so im not sure where to start. Been reading through the CotC threads but still confused with some questions so am hoping for some clarification and guidance 💕

1. Been seeing the term rainbow team, is it a late game content? Is rainbow team the eventual goal to work towards?

2. Who do I select?

My current team is:

Fiore, Shelby
Locke, Ramona
Cyrus, Z’aanta
Hasumi, Cerna

Unused 5-star:
Alaune, Serenoa, Heathcote, Nicola, Olberic, Joshua

Any comments/feedback is greatly appreciated 🙏