You can sand it down and use spray paint. KH3 is just black, so it's relatively easy to match. Sand with a higher grit, maybe 800, you're not looking to sand off anything, just get the surface rough enough for paint to stick.
Why are you so against a shop? It's not that expensive for a grill, maybe $100 at a lot of shops and it will match. I paid $80 to have my grill done in KH3 and it matches perfectly. Spray paint will never look as good as having a shop do it. And it takes a long time to properly sand the grill down. It's also an area prone to chips, so you really want a good paint job on there.
If you don't want to go to a shop, maybe try plastidip. I personally hate the look, but it's much easier than using regular spray paint, no sanding needed. If you don't like it, just peel it off, you haven't ruined your grill.