Paint the mortar-line color first across the entire surface. Take care to 'dress' any paint that bubbles or floods the grooves so the sponge-painted tile color won't contact the mortar paint down in the grooves.

The best sponge-paint finish is when the paint is lean on the sponge, the sponge-taps are dainty, the application is thin and transparent, and some of the mortar color shows through.