Once upon a time, Hiranyakashyap, the King of Demons, won over the kingdom of Earth through years of prayers.
Impressed by his devotion, Lord Brahma granted him a boon: neither at day nor at night, neither in the sky nor on earth could any man or beast kill him, and no weapon could ever destroy him.
This boon made Hiranyakashyap invincible. He became so egoistic that he commanded everybody in the kingdom to worship only him. However, his own son, Prahlad, continued to pray to Lord Vishnu and this devotion increased day by day.
Prahlad refused to worship his father. Hiranyakashyap tried to convert his son's devotion but was not successful. He lost his temper and decided to kill his own son.He ordered his men to throw Prahlad amongst venomous snakes; crushed by elephants; and pushed off a high mountain.
All these attempts were not successful. Hiranyakashyap finally asked his sister, Holika, to enter a blazing fire with Prahlad. The fire slowly consumed Holika, burning her to death. But Prahlad remained safe!
Hiranyakashyap decided to kill Prahlad himself. Prahlad was tied to a pillar and his father hit the pillar. In this state, Prahlad began to chant Lord Vishnu's name. Then a miracle occurred! As soon as Hiranyakashyap hit the pillar, the pillar broke…
Lord Vishnu took the form of Narsimha, a creature that was half-man and half-lion. He came out of the pillar, dragged Hiranyakashyap to the doorstep, pulled him onto his lap and killed him with his nails. Prahlad, who kept chanting the name of Lord Vishnu, came out unharmed.
Prahlad was blessed by Lord Vishnu and crowned as the king in place of Hiranyakashyap. He ruled the kingdom with utmost fairness and righteousness.
So that's why Holi is celebrated as a festival of victory of good over evil. Holi is named after 'Holika' and is the festival of colors.