Here's some information for you Lori!
This isn't using a sponge, but I think it turned out great!!! https://www.hometalk.com/43357650/how-to-paint-tile-to-look-like-old-stone
Are use a paint roller that is meant to look like a sponge. The key is to not use extremely contrasting colors, keep it very subtle in the contrast.
The corners will have to be hand stapled in with a small paintbrush.
Kinda depends on the effect you are going for but here are some videos.... https://www.youtube.com/results?search_query=how+to+sponge+paint+a+wall+with+two+colors