or if you don't want to use a separate program:

• create a terrain
• go to the terrain editor change the terrain size to whatever size you need, change the fractal to ridges or warped ridges and apply it
• if you like the fractal, hit control-c to copy the grayscale image into memory, leave the terrain editor and delete the terrain
• click your light, go to the texture editor, then the image editor and hit paste on the first and middle boxes

Voila! quick and easy brycean light gels. Caustics Generator is a pretty cool little program, though.