One should decorate the body with tilaka, which is the sign of the Vaisnavas. (The idea is that as soon as a person sees these marks on the body of the Vaisnava, he will immediately remember Krsna. Lord Caitanya said that a Vaisnava is he who, when seen, reminds one of Krsna. Therefore, it is essential that a Vaisnava mark his body with tilaka to remind others of Krsna.)

From NoD ch. 6