You can place a plane in front of a light source and in the pigment clause
use an image_map, setting 'filter all 1' as an option on the image_map.
The light shining through the plane will then pick up the colours of the
png "plasma3.png" // the file to read (iff/tga/gif/png/jpeg/tiff/sys)
filter all 1 // N=all or color index # (0...N), V= value