I've taken a different approach to inserts - I've built "inserts" into the the main grille mold, so additional inserts wouldn't fit. The molded-in-place inserts in both the slots and the headlight openings are clearly visible in this photo. Every grille that comes out of the mold will have these "inserts".
My idea is that if this grille got into production, it would be molded in black, so it could be used as-is. If different color inserts were wanted, they could be masked and painted either silver or gunmetal gray (the two factory colors). And of course the main grille could be painted body color as well. I plan to paint mine body color, and I'll paint the inserts either gunmetal gray or silver but I haven't decided which color to use for the inserts yet.
Here's a factory JL grille with silver inserts:
To keep with the factory look I'll be putting a mesh behind the slots. I've found a mesh that looks pretty close to the factory mesh. I've ordered some and it's scheduled to arrive on Friday.
BTW I've designed the molded-in inserts so they'll be easy to mask and paint - 3/8" masking tape applied with one edge flush with the surface of the grille will provide a perfectly aligned mask line for painting.