I believe it has to do with the gamma settings on the Mac monitor which I believe can be adjusted.
It can be corrected for on the developers end also (with effort-time), but I wonder how many non-mega sites are taking this into consideration. As you found out 90% or so of the world sees red and green as intended, yet you saw black on dark red.
If the Mac’s Gamma is altered across the board for all applications I would assume you will lose a lot of the slickness that attracted you to the Mac in the first place.
I’m surprised no-one has written a plug-in for the Mac browsers that will ensure that just coded page content, flash, and pics render in the browser on the Mac Screen like a non-Mac. I assume this could be marketed as a browser enhancement for $$$. Call it something cool like Leopard Fangs.