The ones that I have seen used the backpainted glass match.
I am so jealous - I wanted to do this but just couldn't make it work - Pictures!!

Unlike many of the decisions we make in our kitchens, this one is easy to reverse if you change your mind, which is always nice. I think backpainted glass either in white or maybe a subtly contrasting color (light gray, maybe gray-blue, etc.) would look good. It could also be fun to use some very modern metal covers like these:

Here is a link that might be useful: Modern outlet covers

I did the matching back painted ones to match the backsplash.

I wanted this particular look so much when I did my kitchen that I stripped the mirror off of mirrored glass switchplates. Clear glass ones for back painting were not available at the time (Stripping the silvering off mirrors is a nasty job).

I would do the matching ones all over again.