At present, the rendering plugin does not have the capability of rendering RGB data. It can only deal with data bricks that have 1 number stored per voxel. Color comes in by indexing, through the "pbar" (the color thing next to threshold slider). And, of course, this currently has a limit of 20 color panes.
So, at this time, you really have no option but to discretize your dataset into 20 colors (perhaps cleverly calling them 1,2,3,...20 - 0 will not be overlaid). Then you can load the pbar's color map (unfortunately, this isn't easy in the renderer).
Adding true RGB rendering to this plugin is a non-trivial step. I suppose the simplest thing would be to render each color plane separately, then fuse them. The rendering functions (written by me) do projections of scalar values. They could be modified to allow projections of vector values, but that would be like real work. And for someone who is trying to criminally steal my best employee. Perhaps you'd care to comment.
bob cox
