According to the old UI, there used to be a circle with a certain color on how difficult a level is. Like FSC, you do it in proto, it has a purple eye. I think everybody forgot about that. My friend Atrumvindex has found out about it, so credit goes to him for discovering it.
Devs, please bring back the depth difficulty indicator.
Just adding on, maybe there should be a guide in the help menu about them, for the longest time I had no clue what any of them meant, other than green...