I used the same technique that I employed on an ill-placed light switch plate in my living room last year.
For my red wall, I used a piece of cute scrapbook paper from Jenni Bowlin that is printed with old soda water labels and was the perfect shade of deep red.
This truly is a five-minute project. It takes longer for the Mod Podge to dry than it does to put the whole thing together, and that includes finding a tiny screwdriver to remove & reinstall the outlet cover, and digging the Mod Podge and a brush out of your craft closet.
The outlet cover is still visible, but now it's not quite so obvious, and I love the tiny bit of extra detail that the vintage-style paper provides. It's nice to add a bit of flair to something as utilitarian and ubiquitous as an outlet cover!