The Mysterious “Save For Web” Color Shift by Doug Avery has some great tips for preventing annoying color shifts when saving for the web in Photoshop.
This isn’t a simple set-and-forget process and it’ll require some experimentation, but it’s worth the effort. My images fared a little better with the article’s advice, but after trying the “just use sRGB all the time” method recommended by MacMojo in the comments, colors seem to save perfectly now.