Question with multiple 3d plot colors

[Enrico Segre]

AFAIU that should be possible only if you first transform your surfaces
to  facelet representation (see help genfac3d) and then either you call
plot3d(xf,yf,zf) lumping all the facelets together, or
plot3d(xf,yf,list(zf,colors)) for each one of them, specifying at hand
the colors of each piece as function of z 
(i.e. colors=1+(zf-zmin)*(ncolors-1)/(zmax-zmin) ).

hth, Enrico