After spending the evening with some work on automatically generated images, I had the idea to switch of anti-aliasing (looking, if some font would look better that way), which turned out not to be quite so easy.
Actually you have to use the negative of the desired color to switch of antialising. I include the corresponding line from my code (line split up):
// USE NEGATIVE OF DESIRED COLOR TO SWITCH OF ANTI-ALIASING
ImageFTText ($neuesBild,$fontsize,$fontangle,$TextPosX,$TextPosY,
-$custom_fg,$fonttype,$text,array());
