Interesting, I found the following expression at the About.com forum:
the term "kou itten" [a single spot of red] refers to a single woman among a group of men. Vice versa "koku itten" [a single spot of black] refers to a single man among a group of women. I am not sure if this phrase is also used in formal language, but it seems to originate from a famous Chinese poem that depicts a single crimson flower blossoming among a myriad of green leaves.
Generally, red is associated to female gender, black to male (for instance on public bathroom doors etc).
I'm not sure whether this is some sort of parody or not. Asked at home (yes, my prime resources for things Japanese, haha), she said it's a valid expression.
I'll keep digging.