I know it was an after thought of the customer but.....
The easy way to do have done that grille would have been spray white base coat on the center of it letting the overspray get on the outside of the center. Let it dry real good, then mask the center of the grille off, then spray the body color letting flash between coats, let flash a final time. Pull the paper off the center of the grille then clear the whole grille all at once.
I know Bax has it covered but for others that might want to do it the right way from the beginning and the way Baxs' painter would have done it...
The easiest/cleanest way is hang the grille up.
1)(I missed this step in the reply) Apply body color or black to the back of the grille.
2)Wait for it to dry
3)Backtape the grille openings and the backside of the grille with paper
4)Spray the white in basecoat and let the overspray get on the rest of the outside of the grille.
5)let the white dry
6)mask the white with fineline and masking paper
7)spray the body color on the outside of the grille
8)let dry
9)unmask all the tape and paper
10)clear the entire grille
Does that make sense? That is the way I would do it but other ways are fine, just more work.