Gliders are suppose to be grey. This way they would blend in with the Eucalyptus bark. Typically a grey glider is healthy and when they are grey with brown spots or turn brown they are on a bad diet and this should be addressed immediately.
There are such things as red gliders but they typically come out of pouch a tone of red. The Red Series of gliders include lions, cinnamon's, brown beauties, and buttercream's. They stay a warm color their whole lives.
http://www.petsugargliders.com/colorbreeding.php
The only gliders I know off that change color are platinums. They "powder out" so to speak. Platinum gliders are not red or brown but mostly white in color.
Diet effects gliders a lot and it is very obvious in their fur. Some gliders in the wild will be brown toned sometimes due to food sources but this is just monetarily.