Again, LTA should not be in a 10 gal tank. Go with a hostable coral if you really want to have them in a 10 gal. A 10 is plenty of room for just a pair of clowns, but nems need much more space. LTA can get over a foot.
Color (as far as I know) has nothing to do with pairing. They need to establish which will be female, and she will be bigger. Give it time, it doesn't happen fast.