I painted 4 of the paintable on-wall M3s just this past spring. If you look at my theater build thread and go towards the end, I start talking about it, and show several pictures as I went through the process. Nick's v4 Build Thread
Go to post #417781 which was on 03/16/2016 for my first talk about it.
Looking back, it worked out great, however I was trying to match my ceiling's flat paint, which is prone to scratching for the first several days.
If I wasn't trying to match my ceiling color, I would have gone with a satin enamel paint maybe. Something with a hardener in it, a little more sheen to the look, etc. It would be a lot more durable.
I used a HVLP sprayer, which was easy to use and gave a great even coat. I went for an ever so slight texture, but a little Floetrol in the paint would yield a smooth finish.
I can't speak to the ability to paint the cloth grills though, as I left mine black...
After (and mounted):
I wish that I had more close ups of the finished product, but these were snagged from my post.
Here is a close up of one of the backs showing the super fine texture that I wanting.