Stars and Tiers can not increase, it means can not be upgraded.
1 Star = 2 Tiers
2 Stars = 3 Tiers
3 Stars = 3 Tiers
4 Stars = 4 Tiers
5 Stars = 4 Tiers
Each Tiers has maximum level to go to next Tier.
So, a suggestion is not leveling up 1 star heroes, also leveling up 2 stars only if we do not have that rainbow color.