Differences between cylinders

I first looked into Radiance about 4 years or so ago, and was very
impressed by it, but I've only really got a clue how to use it in the
past year and I'm still not so comfortable with it. Either way, I don't
think I've posted to the list before(*), so hello to you all!

Well, I'm currently using Radiance (not the very latest version, but
a recent one) to render sprites for a game. It's mostly gone well,
and I now have a complete set, but I could do with making improvements
to some of them, and I cant come up with anything more myself now, so
I'm hoping some people will be able to help me out a little.
(I'm posting each question separately, to avoid either getting lost).

OK, the game uses simple tiled graphics, but different tiles need to
match up. The main ones are for a snake. I created geometry for all
the different tiles for it: the body and tail of the snake are made
of cones, and spheres, some created by genworm. These are supposed to
match up, and they do quite nicely.

But the head of the snake was created in a polygon subdivision modeller;
its neck was made from one of the modeller's "cylinder" primitives, which
I smoothed a couple of times so it was reasonably close to a real one.
I exported it to a .obj file and converted it with obj2rad. Used a
little stub file to xform it to the right scale and move it into place.
It should have reasonably matched the cone primitives used for the body.

When the final images were rendered, the neck of the head tiles lined up
properly with the body tiles, BUT, they looked like they were a different
tone. Both were brownish, but the head images were subtly (but quite
consistently) paler. Yes, they were using the same material!
As I'd produced the head tiles last, I'd already experimented with
the effects of running my images through pcond (and had given up on using
flags like -s and ended up just using the -e flag to give the exact same
adjustment to each image). So I'm pretty sure it wasn't that. The head
images had been rendered with a purple background (for chromakeying)
unlike the body images, but I tried replacing that with the exact same
background the body tiles had. Still the head images were paler.

Well anyways, I could get away with leaving the head tiles paler, as
it's not *glaringly* obvious, but it's still visible and annoying. So
if anyone can shed any light on why this might be happening, or what
else I could try, it'd be greatly appreciated.

Thanks in advance,
Tom Barnes-Lawrence
(* -although I can't help feeling I've said all that somewhere before...)