A composite (tooth colored) filling is used to repair a tooth that is affected by decay, cracks, fractures, etc. The decayed or affected portion of the tooth will be removed and then filled with a composite filling.
Natural tooth-colored fillings “composite fillings” were created as an alternative to traditional silver “amalgam". However, current technology in composite fillings has allowed them to be used in situations where in the past only amalgams were used. Composite fillings are not only aesthetically pleasing, but are very strong and durable. They are bonded to the tooth and avoid the use of mercury.
In the past, when an old filling wore out or you had a cavity, your only option was to have a silver filling. Silver fillings have some drawbacks. The edges of the fillings undergo metal fatigue and begin to break down. As they age, they sometimes break in the middle. When fillings break, they no longer seal the tooth. This lets cavities get started again. Silver fillings just fill holes; they don’t add strength to the teeth.
In addition, they expand as they age, so your tooth may break. The silver fillings cause unsightly stains of the teeth and the gums. Now, with recent advances in materials and new laboratory techniques, we have a new filling material that avoids these problems. 
Reasons for composite fillings:
How are composite fillings placed?
Composite fillings are usually placed in one appointment. While the tooth is numb, your dentist will remove decay as necessary. The space will then be thoroughly cleaned and carefully prepared before the new filling is placed. If the decay was near the nerve of the tooth, a special medication will be applied for added protection. The composite filling will then be precisely placed, shaped, and polished, restoring your tooth to its original shape and function.
It is normal to experience sensitivity to hot and cold when composite fillings are first placed, however this will subside shortly after your tooth acclimates to the new filling.
You will be given care instructions at the conclusion of your treatment. Good oral hygiene practices, eating habits, and regular dental visits will aid in the life of your new fillings.