The yellow colour in onion is due to the pigment

The yellow colour in the onion is due to the pigment Quercetin.
Quercetin is the pigment that adds colour to many fruits and vegetables. It is found mainly in the skins and leaves of the plant. Its production is stimulated by the impact of light. It is a powerful antioxidant.
The total content of quercetin in Onion is 300 mg/kg, which is considerably higher than in many other fruits and vegetables.