In Gmax, using NwMax 0.8b6, you set the selfillumcolor on the trimesh modifier. I am sure the method is similar on other versions of NwMax.
Self Illumination can be mixed with opacity values, either on the model or in TXI files, but if you mix it with animated textures, it may overload the drawing algorithms, even if the glowing and animated section is tiny, so avoid doing that combo. This outcome may be video card dependent.
For more details, check out my gems from the June CCC. They combine transparent opacity (as opposed to metallic opacity) with self illumination, and also use a TXI file to set the transparent objects to draw with additive blending. Additive blending lets you make things even brighter, but if used properly, can also help you make a very dark, but noticeable glowing object, like shadow and ethereal stuff in other games.
Basically any non-black self illumination will cause creatures to glow in the dark. The brighter the color, the more glow. To keep it low, I suggest colors with values less than medium gray (0.5,0.5,0.5). Colors with luminance brighter than that can overwhelm the underlying color in dark places.
I believe you will find that when an area is flagged as interior, the creature glow will be more pronounced. Any exterior setting, even lit only by torchlight, will be much less able to show the self illumination. I assume this is inherent to the engine and cannot be modified.
