Q: The color/brightness/tone of my image changes when I use Save for Web. Why?

A: Photoshop is a color-managed application. The Web is not. When you view an image in Save for Web or ImageReady you are viewing it without color management and see it how it will appear when uploaded to the Web.

To minimize changes, Convert your image to sRGB (i.e from Edit menu choose Convert to Profile )before using Save for Web. You should also make sure that Save for Web has been configured so that images are displayed using the document color profile (see attached screenshot for guidance).

You should also ensure that monitor properly calibrated.