Q: The color/brightness/tone of my image changes when I use Save for Web. Why?
A: Photoshop is a colour-managed application. The Web is not. When you view an image in Save for Web or ImageReady you are viewing it without colour management and see it how it will appear when uploaded to the Web.
To minimize changes, make sure that you either have your RGB working space set to sRGB (Edit>Color Settings) or you convert the image to sRGB before using Save for Web.
You should also ensure that you have a good monitor profile.
You can always check what the image will look like in Save for Web or ImageReady by setting your Proof Colours to Monitor RGB.