I had always just painted the whole thing and do a red stripe.
This is once when I had kept the stripe when redoing the black part, so it needed touched up.
There's one of them on the car, they were a little dirty though.
My opinion would be to just paint them black and leave a silver lip/stripe. If you paint over bare aluminum, make sure you have conversion coating or you scuff them and clean them directly(seconds) before primer. that small painting tip helped me tremendously the last time I painted my rims and they held up perfectly to all the rocks and everything I could throw at them.