If you want the background color, just make the text darker. All the other error messages don’t have a special bg color, presumably, so this one is different anyway.
In addition to Ralph’s advice, I’d recommend downloading the Colour Contrast Analyser. It not only allows you to test contrast for people with normal vision, but tests against various types of colour blindness, too. I don’t want to spend time building a site if folk can’t read it when it’s finished, so I find this tool invaluable. (Fortunately, the Windows version runs on Linux under Wine. :))
Well, depending on the judge, you might see a bit of Cat-o’-Nine-Tails action in Designer’s Purgatory, but that’s preferable to the horrors of eternal damnation in Accessibility Hell.