What is the best color to differentiate male and female?

In my app, when a user doesn't provide his / her avatar, I would like to display a colorful circle with the capitalized first character of his / her name. Let us assume, that if an avatar (face miniature) is provided, the gender is obvious.

It is important for users to recognize the gender at first glance, as it is on a search results display (a lot of records) and the app relies on later face-to-face human interaction heavily (they travel together).

Is there any specific, proven background color (for the background of person's first name initial) I should use to differentiate male / female, or, if the first name of the user is provided alongside his / her avatar I should not bother to do so?