It looks so unesthetics when White teeth Looks Brown.

Brown areas on teeth may be a sign of ‪#‎tooth‬ decay ( ‪#‎caries‬ ) or‪#‎enamel‬ demineralization. They may also represent superficial stains—like those produced by ‪#‎coffee‬, ‪#‎tea‬, ‪#‎wine‬, ‪#‎tobacco‬ and food additives. Failing ‪#‎dental‬ restorations (e.g. ‪#‎fillings‬, crowns) often pick up stain at the tooth-restoration interface, or "margin."
Brown spots which appear deep within the tooth are known as intrinsic stains, and are often the result of treatment with antibiotics while the teeth were forming. Intrinsic stain can also be produced by excess fluoride consumption and events that interrupt the normal formation of tooth structure—like childhood illness or injury.

They are also frequently due to tar, an ingredient in ‪#‎cigarette‬ ‪#‎smoke‬. Each of these is considered a source of extrinsic stain, and is generally removable through dental prophylaxis, improved hygiene, and avoidance of these types of compounds.
Brown spots on the teeth may also be due to poor oral hygiene, and/or the presence of tooth decay (caries).