tutorial on this in 3D world a while back, I have it in my folder of saved pages for Vue ;)
used a layer, distribution function as folk say, with a rainbow colour for oil slicks too

"I'd rather be a Fool who believes in Dragons, Than a King who believes in Nothing!" www.silverblades-suitcase.com
Free tutorials, Vue & Bryce materials, Bryce Skies, models, D&D items, stories.
Tutorials on Poser imports to Vue/Bryce, Postwork, Vue rendering/lighting, etc etc!