While Soto's Appliance + Porcelain Touch Up is durable and long-lasting, frequent exposure to moisture will wear off any paint eventually. If the area or number of tiles is small, you may be pleased with the results, provided you wait for it to fully cure before frequent water exposure (more information here). If the tiles are larger, or you want to change the color of a bigger area, a tile refinisher will give you a better, longer-lasting result.
If the tile is smooth, paint may have trouble adhering to it, so lightly scuffing the surface with sandpaper may be necessary for optimal results. And as with all painting projects, make sure the surface is free of dust, debris, oil, grease, etc., and is clean and dry prior to painting.