How big does a red empress need to be before you can tell it is a male or female?

I have 2 red empress' but not sure if they are male or female. They both look like juveniles, but the larger one is between 3-4" and is starting to get orangish lookins in its dorsal fin, fin, and tail fin. Is that one a male? The smaller one which is about 2-3" doesn't have any coloring. Looks like a plain juvenile. Here is a link of what a red empress looks like steve a href="http://www.aquahobby.com/gallery/e_Protomelas_taeniolatus.php" rel="nofollow"http://www.aquahobby.com/gallery/e_Proto…/a 35 yr exp doubt it if you dont know what a red empress is.