Just as white clothing can be prone to stains, regular use can discolor a white Casio Baby-G watch. Keep the watch and your wrist dry to help stave off yellowing. If stains have already set in, you can fight them with tools that are easily retrieved. Note that different parts of the watch require different cleaning methods.
Moisten a soft cloth to wipe away stains on the watch itself. Dip the cloth in water mixed with a gentle detergent before wiping the watch's bands.
Scrub jewel and metal components of the watch with a brush and well-diluted soapy water.
Battle yellowing with three percent hydrogen peroxide. Spray the solution onto the yellowed areas of the watch, and let it sit for a day before scrubbing at the stains. Repeat this process for a few days until the yellowing vanishes.