Hanuman was greatly devoted to Lord Rama. A particular incident was when Sita put sindoor on her forehead, Hanuman asked her why. To this, she replied that since she is the wife and companion of Lord Rama, the sindoor was a sign of her unconditional love and respect. 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 worshiped Lord Hanuman in the future with sindoor would see all their difficulties go away.