[tripzilch]

In graphics design I usually pick the colour that contrasts most with the background, and use a text-shadow (looks better than an outline, anyway) in the other colour. Use none or very little shadow blur.

What you should most definitely not do, is give each character (or word) its own colour depending on the background. As others pointed out, this would be a pain to implement, wrapping everything in spans etc, but most importantly it also just looks terrible.

Though the very first step is to simply try and see if you can avoid the situation entirely :) [which may be harder if the images are user supplied]