So not really an issue (I hope), but sometimes my BN are darker in color - like very dark, I can't differentiate between them and the dark rocks in my tank - on the other hand, sometimes they're very light. I heard that it's normal and it has to do something with whether they're hiding in caves a lot or not but... is it? Does it mean anything at all? Mood? Health? Either color preferable? Pretty sure my BN are the common btw, but I don't really know how to tell different BN species apart.

colour is affected by a lot of things including stress, diet, substrate colour, hiding, mating and more.

unless they are constantly pale then you are just looking at normal variation in colour. if they are alway light then it copuld indicate an issue.