You're right about the coloring but, yes, it can be tricky to differentiate between males and females as they only color up once you've brought them home in many cases (particularly if those at the LFS are juveniles). You can also try looking at body shape (deeper-bodied females, wider when viewed from above) and fin shape - longer dorsal and anal fins in males. It is rather difficult though.
Hi Gill, congrats on all the nests.
just quickly, can you tell me how you vacuum your gravel when it's so fine without sucking it all up? I have the same stuff...