I believe the spots you are referring to are kind of like lighter patches? my 2 commons had this until i added drift wood to their tank and now its gone.
one of my commons did that it was when i removed all the decor and replaced it with new he regained his colour once he found a piece of bogwood to live under
Looks like stress marks, bits go lighter when they are stressed out, mine do this sometimes during water changes, if water params are ok then it's nothing to worry about