Operculum color has nothing to do with it; to much variation in the species as a whole. Colour patterning is also unreliable as there is both hormonal and "mood" control over this feature. Shape of the unpaired fins is reliable but only works consistently with fish of similar ages; it is more of a gestalt thing when you have several to compare. I had a pair for over a decade and the female's fin extensions would make you think she was a male (much more elongate than your male's). That is until you compared her side by side with her mate.
Aside from slicing them up and comparing chromosomes, the only 100% reliable way is to watch one lay eggs, she's the female. If you get a hatch, then the other one is male. And yes, 2 females can and will spawn together each depositing eggs. Of course, there will be 0% hatch.
