How can one change color for "z" in SG3d

[Bill Brown]

When you use a map for color in SG3d, it uses the same 
color table used by GRASS as when you do a d.rast.  So
you can use r.colors or if you understand how the
color tables work, edit your color table file for
the raster (in $LOCATION/colr/<rastername>) to reverse
the color ramp.