Ten years ago I experienced the same problem, so I invented a product called Dr. Grout. I now sell this product direct, rather than through retail outlets. It is a white, water-based stain that can be changed to any colour by simply adding a small amount of tinted latex paint. I suggest that shower enclosures should remain white, while you can have your pick of colours for floor grout, a shade darker on the floor is recommended. The product is extremely user-friendly. You paint it on the grout line, and wipe off the excess from the tiles with a damp cloth after it has dried. You can't make a mistake!
For more information, feel free to contact me by email or by phone: 905-764-9150.