Preparing images for the web doesn’t start at saving. It’s a good idea to check out a few things before you go into the Save for Web dialog. First, you’ll want to have a look at your image mode.
When designing for the web or any other on-screen media, you’ll want to make sure your images are formatted in RGB. Traditionally, CMYK is reserved for print. This is because there is a fundamental difference between creating colors with light vs. simulating colors with ink. While printers use tiny dots containing only one of four colors (Cyan, Magenta, Yellow or Black) to create the illusion of other colors, screens are capable of producing millions of colors. Furthermore, RGB is an additive system, meaning the result of greater amounts of color is white.