The potential "problem" with the gray spray-paint, is the possibility of it looking more like overspray or tiny-dot speckled. Granted, if still wet, it might blend-in with the black, but not sure.

The Flat Black High-Temp enamel should protect the radiator, and paint flattens as it ages.

Brown plus gray might look better if the Rusty-Old radiator Water technique doesn't get it done for you.

Good Luck with it.