In FF, you can pull the style from the page. It messes up the layout, but does change it to black and white. You can go to View, then Style and choose no style.

I'm sure there's accessibility plugins for FF that will change it to black and white or high contrast for you, might be easier than getting them to change the colors. IE has some built in accessibility options, but I don't have access to IE at the moment to see what they are. Might be worth a look.