Man, I have been trying to figure this out for ages and stumbled across it tonight.
When saving for web, I’ve always had trouble with my images being lighter and more washed out once saved than they are in Photoshop. Well here’s the solution:
View > Proof Setup > Macintosh RGB
Simple, right? This means you’ll actually viewing your files in RGB originally, so no color variation when you save them as such.
Rock on with your web designing self.