Not a revelation, but everything that doesn't match won't match. So imo it either makes sense to concrete and paint a similar colour so it doesn't stand out too much, or go full contrasting.
Or if you want something more interesting or contrasting trim the cut tiles back to the edges of the whole tiles and lay some sort of funky mosaic or cut a bit of pine to fit and stain it whatever colour fits in with the rest of the decor.
What do you actually want the extra space for now? Because to me it sort of lends itself to an indoor plant area.
It's a bedroom, likely bed head agaisnt that wall anyway so it'll be quite unseen. Then in a year or two our son will move into that room when he comes out of his nursery, and I'll put a suitable floor for a kid.
Baby banquette?
Not a revelation, but everything that doesn't match won't match. So imo it either makes sense to concrete and paint a similar colour so it doesn't stand out too much, or go full contrasting.
Or if you want something more interesting or contrasting trim the cut tiles back to the edges of the whole tiles and lay some sort of funky mosaic or cut a bit of pine to fit and stain it whatever colour fits in with the rest of the decor.
What do you actually want the extra space for now? Because to me it sort of lends itself to an indoor plant area.