The Macbook's white exterior doesn't have the same luster when it's grimy from months of constant use. Contact with hands and the sort of detritus that accumulates within a laptop bag can leave the white Macbook looking grey or yellowed. Before cleaning your Macbook, turn it off and unplug it.
Clean With Care
Use a soft, lint-free cloth to clean the exterior of the Macbook; a cotton or microfiber cloth works well for the task. Don't use any hard solvents or abrasive materials, which can scratch the exterior or dull the Macbook's shine. Lightly dampen the cloth with water, or a mixture of half water, half vinegar. Never spray or pour liquids directly on the Macbook. For sticky messes, a small amount of rubbing alcohol can remove the worst of it; use a dampened cloth to remove the sticky mess, then a clean cloth to wipe the surface and remove any excess rubbing alcohol.