Unfortunately, I haven't been able to figure out how LRR determines which colours are supposed to be transparent. The closest thing I've been able to do is create transparent particle effects which works by treating pure black (000,000,000) as fully transparent, pure white (255,255,255) as fully opaque, and everything else inbetween partially transparent. Then, in the Surface Editor in Lightwave Modeler, you have to go to the Advanced tab and set Additive Transparency to 100%.