Chefs wear white uniforms because they help them keep cool in hot kitchens. White also indicates cleanliness, partly because it's hard to hide food stains on pure-white fabric.
Kitchens are hot places to work, and white reflects heat better than any other color. Working in a hot environment means that every degree counts. Kitchen whites are generally worn by head chefs rather than by all of the kitchen staff.
Marie-Antoine Careme, a popular chef in 19th century France, is said to have been the inventor of the white chefs' coat. He deided to create a uniform specially to honor chefs and chose to make the coat white as a symbol of cleanliness. Another chef brought the white coats to London, where they quickly caught on. The white coats are now widely used in commercial kitchens all over the world.