How does one color superimposed stuctures?

[Dan Strahs]

 I have several structures I would like to superimpose and graphically display
 with a ribbon backbone. Quite simple, really, and a variety of programs
 will readily perform this task.

 However, I would like to blend the colors of the superimposed backbones,
 such that when the ribbons of the different structures merge,
 their color merges. For example, if I display two similar proteins with
 a blue ribbon and a red ribbon, I would like the portions of the ribbons
 that occupy the same space to be assigned a color of purple with a
 brightness/saturation approximately twice that of the individual protein's
 ribbons.

 My question is:

       Which software will do this? Insight and Midas don't appear to
       have this ability. Does Quanta? Is there another program, such
       as O, Grasp or MacroModel that does? Am I wrong and can Insight
       do this task?

 Thanks for any help.

                                       Dan Strahs
                                       Department of Physiology and Biophysics
                                       Mt. Sinai School of Medicine
                                       strahs at msvax.mssm.edu