There are 7 levels of reward chests:

They are colored and rated just like the rarity of the players

NOTE the chests can pay out single "items" like below chart OR payout more than 1 thing and be a mix of things. E.g. can a legendary chest contain e.g. 60M and a epic player instead of 100M cash.

Grey/common chest = pays out 1-10 credits or 0.1-1M cash or a common player card

Green/uncommon chest = pays out 10-50 credits or 1-2.5M cash or an uncommon player card

Bronze/rare chest = pays out 50-200 credits or 2.5-10M cash or a rare player card or a small training camp

Silver/super rare chest = pays out 200-500 credits or 10-50M cash or a super rare player card or a medium training camp or staff booster

Gold/epic chest = pays out 500-1000 credits or 50-100M cash or an epic player card or a large training camp

Red/legendary chest = pays out 1000-3000 credits or 100-300M cash or a legendary player card

Purple/mythical chest = pays out 3000+ credits or 300M+ cash or a mythical player card