Probably a female but could also be a juvenile male that isn't colouring up because it is being bullied by a bigger male or is isn't mature yet.
Look and see if the fish develops a dorsal blaze. This is a coloured line that runs from the start of the first dorsal fin to the top of the mouth. It is either yellow, white or pink, depending on species. Males produce the dorsal blaze when showing off to each other and to females. The females do not produce the dorsal blaze.