Hello E Mgl! You can turn this space into storage, but the question is: do you need it? Otherwise, there is no point in tiling all the way down under the counter top. Hope this is helpful!

If you prefer to add tile at the bottom consider using a matching schluter edge there and no baseboard.

I do not need storage there but maybe a long shelf as decoration.

We decided no tiling. What about painting there dark gray? Is it ok to keep there as is?

The vanity would really float off the wall if it was the same color underneath.

I think it would look better if you continued the tile. At least match the color if you don't do that.