That “spectrum” texture is nothing more than a diffuse map. MMD allows you to set up various proxies and the author of this ring set them all up to be colorable. There are various (and potentially better) ways you can do this, however in Source you could just leave the ring white and set the ring to be colorable directly in-game with the color tool or via console proxies if it’s attached to a model. This should work fine if the gem is a solid white (perhaps with a cubemap or phong settings to tint the shine subtly based on color) and the body of the ring is not colorable.
If you use a special specular map, you can make the ring appear to only have shine in certain areas which give it the appearance of catching light in certain areas when it really isn’t (see jacket below).
Again, phong can be tinted any color you want.
Just for the rest, tr.spa is the cubemap used on the gem and m.png is the shader ramp (these translate to $envmap and $lightwarp in source, though I don’t think you can convert spa files to usable textures in Source).
For the ring, s.sph is the cubemap and s.png is the shader ramp (see $lightwarp again for Source). The diffuse is just a solid color tinted at [0.8, 0.8, 0.8] though the diffuse is overridden by the cubemap. For Source you can just use a really dark gray with a cubemap over it to get a decent effect.