As it says, thrusters can't be recolered. I'd say making the white take the primary outer hull color and the black the inner hull color would be the easiest way to set it up? (some silvery specular stoff for the small nozzle would be neat tho)

On a minor note, the thruster box has some z-fighting with the bottom of the decoupler going on.
Veeeeeery minor thing.^^

I think this is a duplicate of #625

In fact, it is ;)