From BlenderWiki

Jump to: navigation, search

[edit] How do I invert colors of my image/texture/etc. ?

You can use Mix node found in Add\Color\Mix to do this. Set Mix node to Subtract mode and set the Fac value to 1.0. Make sure that the upper color of image inputs is white and put the image you want to invert to the lower input. As a result of this you should have inverted image in the output.

Retrieved from "http://wiki.blender.org/index.php/Doc:FAQ/Compositing"