Select a liquid grout pigment that matches the countertop. If your countertop is multicolored, match the middle color in the countertop. Mix the pigment with a light-gray seam grout in a container until it matches the intended color.
Smooth grout in between the seam line with a plastic scraper. Push the grout into the seam to fill the opening completely.
Wipe away any excess grout from the countertop with a wet sponge. The seam should become harder to see.
Purchase a seam sealer compatible with your countertop (e.g., granite or Corian) and the color of your countertop.
Squeeze the sealer into the seam of the countertop.
Wipe away any excess sealer with a wet sponge. Then allow the sealant to dry in place. The seam will become harder to see.