SFGate

Manon Floreat Depending on the other elements you have in your space, you could do just about anything. Reclaimed wood may appeal to you if you want to incorporate a rustic element in the space.

River House

Family Room illuminated niche wall

Stainless steel or glass would add a little sparkle with lights you'll add.

2012 New American Home

Coralee Angell I suggest finding a patterned wall paper in your accent colour (red?) and install it on the back of the alcoves. I would then install a couple glass shelves in each which can be used to display family photos.

carolannr I think if don't plan to put shelves in them, the bottom could be wood similar to the flooring in the room.

Carolyn Albert-Kincl Design If your niches are painted, they do not have to have a shelf. You can use them just as they are without another color or material interfering with what you want to place in them. Your niches look the perfect shape for a pair of ginger jars. They come in lots of different colors, in solids or patterns, and in all sizes.

Carolyn Albert-Kincl, ASID

twylahaj If you are using them to highlight art of some sort (glass, sculpture, etc), I wouldn't paint them. If you are wanting shelves to be able to display items, glass shelves would allow the light you are installing to pass through, which would be lovely. If by shelves you mean just on the bottom as a finished surface, something shiny would reflect the light back up, which would also be nice. If you're installing lights it seems to make sense to get the most light and interest out of your investment.