Why is sindoor (vermillion) applied to Lord Hanuman’s body?

Sindoor or vermilion holds lot of importance in Indian society. The tradition of application of sindoor in the parting of hair by married Hindu women is considered extremely auspicious and is being carried on since centuries.In traditional Hindu society, wearing sindoor is considered must for married Hindu women. It is a visible expression of their desire for their husbands' longevity. 

According to a popular belief, once when mother Sita was applying sindoor to her forehead, Hanuman witnessed it and asked her the reason for doing so. She replied that by applying sindoor, she ensured a long life for her husband Lord Rama. The more sindoor she applied, the longer Rama’s life would be. Devoted Hanuman was elated to hear this and being a devotee to lord Rama he then smeared his entire body with sindoor, in an effort to ensure Rama’s immortality. Hanuman then covered his entire body with sindoor to prove his love for Lord Rama. Lord Rama was really impressed by this and granted a boon that those who worship Lord Hanuman in the future with sindoor would see all their difficulties go away. Hence Hanuman’s idol is always coated with sindoor.