Comment 14 for bug 339855

@Jeff: the Python standard library is a treasure trove of neat stuff. For example, searching for "color" brought up http://docs.python.org/library/colorsys.html. There are also lots of cool diagrams at http://en.wikipedia.org/wiki/HSL_and_HSV.

One approach would be to identify a minimum difference in saturation (S) and either value or lightness (V/L); or some combination thereof. Convert the foreground text colour and the tag colour to HS{V/L} and compute the difference. If it's not big enough, adjust S and V/L while keeping H constant. Then convert back to RGB and use for the tag background.

colorsys seems to give S=0 for black, but treating that as S=1.0 might be more useful.

Also, I think this is what you've already suggested, but the border around the tag text should always be the unaltered tag colour.