It's a trade secret! ;)

DecoColor Opaque Red. You can find them at many hobby shops, but here's the Amazon link: https://amzn.to/34D9PJS

You'll want to do two or three coats, letting paint dry for 5-10 minutes in between each coat. You will inevitably bleed over onto the face of the buttons. Just dab this on a paper towel, and wipe away the excess with the moistened paper towel. I did the same process with the Mode button, but I spray-painted the Start button.