High-Tech LEGO Chapter 9: Hacking LEGO Light Bricks
Chapter 9 offers a guide to modifying LEGO Light Bricks to create any desired color. A couple of video examples are below.
LEDs are available that change through colors, such as this example that cycles through red, green, and blue.
One project described is to install a UV LED in a Light Brick to build a tool for testing for fluorescence. This video shows a UV Light Brick passed over a $50 bill to see the fluorescent strip embedded in the paper.