I have a graphics card whose default visual is 8bpp PseudoColor but it also provides multiple 24bpp TrueColor visuals.

Most of color-intense applications like gs or xdvi correctly pick
up a TrueColor visual. Unfortunately, gv seems to work in the
default (PseudoColor) visual and hence provides poor quality for
many postscript files.

I tried setting Xdefaults to

gv*depth: 24
(gives BadMatch error)

gv*visual: truecolor
(gives Warning: Cannot find Visual of class truecolor for display :0.0 and still uses pseudocolor)

and both
(give BadMatch)

Automatic selection of the best visual available on the system, or the ability to select a user-specifid visual, would be an
extremely useful feature!