It's a thin and even coat of copper spray paint. James and Brian did a test spray one of our spare balls to test the theory, when they saw it looked promising they handed it over to me (the prop guy) to painstakingly paint the balls on the set. The key is just enough paint for an even coat but not to thick so the projection comes thru, it was quite nerve racking! The effect is quite stunning giving the best of both worlds, you can leave it off for those that want it to look like TOS or turn it on for a phase II upgrade look. It wasn't fun, many very light coats of paint to get the projection just right!
When we do our Christmas construction crew gathering this coming weekend I'll take a wide video shot of the intermix chamber with them all working.