There definitely seems to be a little bit of weird behaviour with the font renderer on certain background colours. Could be related to the method it uses to determine the perceived brightness, and how it adjusts the text for either a dark or light background.
This is with the main background colour set to 7F7F7F:
And here it is with 808080:
Apart from simply being a bad choice of colour on my part, you can see that the text looks fairly normal against the 7F7F7F background, but against the 808080 background the green text definitely has some anti-aliasing problems.