You can create an RGB-valued dataset via to3d, if your input is a set of colored images (e.g., JPEGs, GIFs, PPMs) created somehow. Alternatively, you can use 3dcalc somehow to compute the R, G, and B components separately as scalar datasets, then combine them (as Tom points out) with 3dThreetoRGB. Note that if you want a black voxel, you do not use R=G=B=0, since that won't overlay in AFNI -- instead use R=G=B=1 (which is pretty dark).

Unfortunately, at this time you cannot volume render such datasets. You can display them in the 2D image viewer windows. I don't think that you can send them to SUMA, either, but actually don't know -- Rick Reynolds is in charge of that part of AFNI now.