Start with a clean basement floor. Paint the entire floor the color you would like the “grout” color. Start painting your base color.
Paint your base color, and right away (before it dries) go over the edges with a contrast color.
These are the colors I used.
Don’t get fussy about the contrast color. The more mottled and uneven the better. If you get a big blotch, you can always go over it with the base color again.
When I pulled up the masking tape, some of the first color came up. I put a tile to mask the newly painted “tile” and then re-painted the “grout”.
Finished! I didn’t get the masking tape quite straight. I am sure you will do better! But if not…it is just a basement floor, I put a throw rug down. People can’t believe it is painted! Next on my plate…paint the far wall to look like brick.