It is important to brush your teeth to have good oral health, but that doesn't always prevent your teeth from yellowing. There are many reasons for yellowing. Some reasons are drinking coffee and other discoloring drinks, colas and tobacco, and just regular aging and wear. Plaque can build up, and if your teeth are not cleaned, the dentin (which is a slightly yellow color) can show through the enamel. The enamel can also be worn away by poor brushing and flossing. To get your teeth their whitest, have the plaque removed by going to the dentist, and then get them deep cleaned. After the plaque is removed, whitening can be done to remove discoloration. If your teeth are suddenly discoloring, it is best to consult your dentist to see if there is a more serious issue.