As far as I can tell Bisca's insignia has never been shown. Not in pre-timeskip, post-timeskip or even her Edolas counterpart. Sorry.
I have looked through her appearances, and even in a bikini or in the bath scene in the OAD "Welcome to Fairy Hills", I haven't spotted her insignia. It's hard to find an area of her body that hasn't been shown. My guesses would either be on the back of her neck (always covered by her hair), the soles of her feet or her arse.
As for the colour, it could be anything. But I like to think she has a dark red insignia.
