Most commercial tandoori pastes contain artificial colouring to give it a distinct bright red colour, but it's only a westernized ingredient adaptation added to appeal to our visual senses. The traditional recipe should only contain natural spices. Artificial colours at worst are known to cause headaches, behavioural disturbances and other complications to sensitive individuals. At best they may have no effect. But why take the risk when you don't have to.
Recipe courtesy of Liz Hemphill from the book Spiceries.