According to Ramanyan, one day Mata Sita was applying Sindoor or vermilion on forehead, at that time arrived Hanumanji and He saw Mata Sita applying Sindoor on her forehead, to His dismay, He asked Mata, why and what is she applying on her forehead,
Mata Sita silent for a while, answered Hanumanji, that she is applying Sindoor on her forehead, doing so, will give Bhagwan Rama long life and He will always be happy in His life and with her too,
Introspective for a while, Hanumanji thought, by applying small quantity of Sindoor, if Prabhu Rama can have long life, so why should He cover his entire body in Sindoor, by doing so Shri Rama will be immortal,
When Hanumanji came infront of Shri Rama, body in Sindoor, Shri Rama after understanding the whole logic behind Sindoor, Shri Rama gave a boon to Hanumanji that He will be immortal and ever existing,
From then, the idol of Hanumanji is offered and applied sindoor by the devotees to keep Hanumanji happy and smiling.
Second reason is, as per Shastra, red color is a symbol of Rajo Guna activeness, speed and energy, for the same, during any sacred ritual, we are asked to wear red cloth,
Thirdly, sindoor is generally made from lead, a powerful source of energy,
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