As you can see below, by looking at the left hand column, I have one 4* troop of each colour at Level 17.
For now I have stopped there and, as you can see, have started levelling other 4* troops up a bit, so I can eventually have 5 4* levelled troops for each mono attack team.
What do other folk do? Do you focus on one troop of each colour to have one of each really strong for a rainbow defence team, or do you level up a range of troops to strengthen any mono attack teams you may use?
Thanks very much