White has become very popular as a background for portrait photography shot in a studio. Sometimes I've seen shots taken where the lighting isn't quite right and the background is a dull grey color or the floor is dirty. Other than keeping the floor clean and getting your lighting right in the first place you can always use Photoshop to fix it!

Open your image & open your Levels palette. Holding the ALT key slowly move the white slider (the one on the right!) in towards the middle. You normally only need to move the slider a minimal amount. You'll notice the background starting to "clean" itself! Don't go too far though because you will start to lose skin tones & other highlights. If you keep holding the ALT key whilst moving the slider you can see your subject starting to get effected too, at this point you've gone too far so back up a bit!! Click ok when you're happy with your adjustment so far.

Next, make a Levels adjustment layer (see set of images in collage below for screenshots) and move your black slider (the one on the left!) in towards the white slider. Now you can see where your background is dirty. Click on your background layer and using the dodge tool set to "highlights" & about 3 - 8% dodge the dirty areas. Once you've finished delete the adjustment layer & you should have a whiter than white background without having effected your subject matter!