Sri Rama proclaimed in front of his court that out of the million good virtues and qualities Hanumanji is known for, his most obvious and unparalleled quality is bhakti/devotion to Lord Rama. Hence, he became the embodiment of devotion itself.
The story goes like this:-
One day while Hanumanji was doing seva/service to Mata Sita, he noticed that she was applying a red colored powder on the area where her hair partitions. When asked what it was and why was she applying it, she replied it was sindoor/vermilion. She told Hanumanji that by applying sindoor, her lord Sri Rama would be very happy and he would live longer. However, Sitaji forgot to tell him that this tradition was practiced only by a wife for her husband.
Hanumanji knowing this information waited no longer to make his lord happy and live longer. He smeared sindoor with some ghee all around his body and went to the court. Without even bothering why people were laughing at him, he went straight to Sri Rama. When asked by his Lord, he answered he had done this to make Sri Rama happy and live longer according to the advice of Mata Sita. He said, " Mata Sita is a Goddess, so a little bit of sindoor was enough for her to make you happy. However, I am just a servant and hence, I needed a lot of sindoor to see a smile on your face."
Sri Rama was moved by his innocent child like behavior and sincere devotion. He proclaimed that who ever dressed Hanumanji's idol with sindoor every Saturday (Lord Vishnu's Day) would get all their desires fulfilled.