I found the solution! I cut a small piece of flat sponge. I brushed metal onto one part of the sponge, thinly. I lay the sponge flat on top of the tile with a quick dab, and slowly dabbed my way across the whole surface of the flooring - leaving behind a perfectly surface-painted floor! No metal in the holes or cracks! So fast! So easy! Only two holes on the whole floor, I think, that needed a quick dab of black paint.
I then painted metal on the wide rectangle border around each grate, then washed the whole floor with badab black to dirty it up a little, with some dabs and streaks of badab black on parts of some grates as well, giving the whole thing a metallic, used, kind of look about it. The badab black also tones it down a fraction, and it also tidies up the black lines that run between each tile.
Before, the metal of the guns would be invisible against the raw metal of the floor. Now the gun stands out a touch more, making the figure more visible against the floor.