You could also emulate noise. If you use, say, 16 grey-scale colors for the picture, and you have 192 palette indices, then each pixel of a particular color could be one of 12 different palette indices (chosen randomly).
Then in script you could modify the intensity of each palette color randomly in each game cycle. An area of one solid color would have 12 different pertubations possible per pixel, so it would probably look like random noise.
The hard part is generating the palette indices in the first place. You'd need to be able to produce 12 variations (in your image editor), where a random 1/12 of the pixels are on and the rest are transparent. Then import them one by one into a pic in SCI Companion, into different palette ranges. It would be trivial to write code in SCI Companion that assigns a color to a random palette index, but it seems like functionality very specific to this scenario... would it have other uses? hmm...