Oscars are the biggest drams queens and they'll change color by the minute based on mood. My tigers change from black when they're angry and almost yellowish when they are stressed or scared.
Oscars are monomorphic which means there is no difference between males and females so you can't sex them till you see them laying eggs. Some people will say after they reach sexual maturity you can tell by their genitals but I never could.
I rescued a pair I thought were both females because I kept seeing eggs but they were never fertilized but low and behold a few months later I have over 300 little oscars that lived. So until you see eggs there's no way to tell.