At Christmas I hung a metal stocking over the firebox to add a bit of color.
A few weeks ago, I was frustrated (again) by the fireplace, and decided to take matters into my own hands. I grabbed a paint brush and made a custom color with odds and ends of acrylic paint. I then brushed several light coats of paint over the dark colored vents. I had to tweak the color a few times to get it right, but I feel like I matched it pretty well.
I love how it seems to brighten up the space and you don't notice the vents that much any more.