The colour themes of my book covers show you if they're in a series or not; I've made a little guide to help:

✖️Dark Purple- Satan Spawn~ Sherlock
(MorMor, Johnlock, Lestrolly, OCs)
➕Light Purple- Origins and Sidestories in the same AU

✖️Red- Rapid Improvements~ Supernatural 
(Destiel, Trans!Dean, Norse Mythology)

✖️Green- By Asgardian Law~ Skyrim/Avengers/Thor crossover thing. Oh man, this will be fun.

✖️Pink-

✖️Orange-