There are various factors that can make your teeth look dull and yellow.

Certain foods can stain your enamel, which is the outermost layer of your teeth. Also, plaque build-up on your teeth can cause them to look yellow.
The discolouration can be treated with regular cleaning and whitening remedies. Sometimes, the teeth also look yellow because the hard enamel has eroded, revealing the dentin underneath. The dentin is naturally yellow bony tissue that is underneath the enamel.
Here are six simple remedies to whiten your teeth

Practice oil pulling

Oil pulling is a traditional remedy that improves oral hygiene and removes toxins from the body. The practise involves swishing oil in your mouth and removing the bacteria that can turn into plaque and cause your teeth to look yellow.
The most commonly used oil for oil pulling includes sunflower and sesame oil. You can also use coconut oil, which is high in lauric acid and is known for its ability to reduce inflammation and kill bacteria.
Studies show that daily oil pulling reduces bacteria in the mouth along with plaque and gingivitis.
Doing oil pulling with sesame oil reduces Streptococcus mutans bacteria, which is responsible for plaque and gingivitis.
To do oil pulling, put one tablespoon of coconut oil in your mouth and push and pull the oil through your teeth. Oil pulling is a safe practice and can be done by anyone.

Brush with baking soda

Baking soda has natural whitening properties and thus is an important ingredient used in toothpaste. It’s a mild abrasive that can scrub away the stains on teeth.
It also helps create an alkaline environment in the mouth, which prevents bacteria from growing. Science has yet not proven that baking soda can whiten your teeth, but studies have shown that toothpaste that has baking soda has a significant whitening effect.
To make use of this remedy, you can mix one teaspoon of baking soda with 2 teaspoons of water and brush your teeth with it. You can do it 2-3 times a week.

Hydrogen peroxide

Hydrogen peroxide is a natural bleaching agent that also kills the bacteria in your mouth. Hydrogen peroxide has been used for ages to clean wounds because of its ability to kill bacteria.
Studies have found that a toothpaste containing baking soda and 1 per cent hydrogen peroxide can lead to significantly whiter teeth.
Another study has found that toothpaste containing baking soda and hydrogen peroxide when used twice a day can lead to 62 per cent whiter teeth in 6 weeks.
It should be noted that there are some questions regarding the safety of hydrogen peroxide. The diluted concentration is safe, string concentration or overuse can cause tooth sensitivity and gum irritation.
If you are planning to use hydrogen peroxide, make sure to 1.5 or 3 per cent solution to avoid any side effects. You can also use it as a mouthwash before brushing your teeth.
You can mix it with baking soda and brush your teeth with the mixture.

Eat fruits and vegetables

A diet high in fruits and vegetables is great for your overall health and teeth. Crunchy, raw fruits and vegetables can help rub the plaque away as you chew them. Though this is no substitute for brushing your teeth. Strawberries and pineapple are two fruits that have been claimed to whiten your teeth.

Prevent tooth stains

While your teeth will turn yellow as you age, you can prevent stains by doing some simple things.
Limit the intake of coffee, red wine, soda and dark berries. These all are infamous for staining teeth. You do not need to avoid these substances completely, but you must limit their intake.
You can also drink beverages that are known to stain your teeth with the help of a straw.
You should brush your teeth sooner after you consume any such beverages.
Avoid chewing tobacco, smoking cigarettes as both can cause tooth discolouration.
Limit your intake of sugar, as too much sugar, can increase the growth of bacteria and cause plaque and gingivitis. Make sure to brush your teeth right after consuming sugary foods.
Have calcium-rich foods like cheese and milk to help protect your teeth from enamel erosion.
Brush and floss regularly
Teeth discolouration is mostly a result of plaque buildup. Regular brushing and flossing can help your teeth stay white and reduce the bacteria in your mouth and prevent plaque build-up.