Microwaves and painting brick

We're a microwave family. I know lots of people who live without one, but I could not, for coffee reheating purposes alone. Our microwave had seen better days (like, I'm almost certain it was a fire hazard), so when I saw this nearly-new one on Kijiji for next-to-nothing, I snapped it up. We didn't want or need anything fancy and I like how the handle resembles our cabinet hardware.

The opening it sits in is anything but pretty, I know. I think the plan will be to paint the interior (and possibly the exterior, but we're waiting until we're done updating the kitchen to make a decision).

The picture in the lower right-hand corner shows you what the opening looked like before. We trash-picked the wood and Adam cut it down to size (this is one of two openings in the kitchen side of the chimney and we found two similar pieces of wood, so, one for each opening). We cut a large notch in the back corner so the outlet is easily accessible. The large wire hanging down is no longer active (we had both openings checked out by an electrician), so I just tucked it out of the way for now.

I think painting the interior will make it less of an eyesore and help the cinder block blend in with the brick. I'm leaning towards a dark colour and possibly painting just the sides (at least until we make a decision about painting the entire chimney). Here's a really terrible Photoshop mock-up to give you an idea, or possibly confuse you further (sorry about that).

What do you think, should I try painting only the sides? Paint the back, too? Do you reheat a single cup of coffee multiple times throughout the day? Or, are you microwave-free and loving it?