The festival of Holi is usually associated with Lord Krishna. In places like Braj, Vrindavan and Mathura, Holi is a grand festival in which people celebrate the eternal love of Lord Krishna and His divine companion Radha.
Lord Krishna has always been portrayed as the lovable prankster in the legends. His playful nature and His 'leela' with the women of Braj is immensely popular. More than anything else, the eternal and divine love of Krishna and Radha make this festival all the more charming. The people of the Braj and surrounding areas enact roleplays during Holi which brings to life the divine legend of Radha and Krishna.
Once Lord Krishna was extremely envious of His companion Radha's complexion. Krishna was dark in colour, while Radha was very fair. So, he complained to His mother Yashoda that nature is very unjust as it had made Radha fair and Him dark.
To pacify her young son, Yashoda asked Krishna to go and colour Radha's face with whatever colour He pleased. So, Lord Krishna paid heed to the advice of His mother and applied colours on Radha's face, making Her look like Himself. Thus, the custom of applying colours to each other on Holi is said to have begun.
This lovable prank of the Lord gained popularity as He also played this prank on other women or gopis of the village. He threw colours and teased them with colorful jets of water. Hence, the tradition of applying colours evolved and became an inseparable part of the festival.
Holi is the celebration of love between Radha and Krishna. That is why there is a custom of applying colours to one's beloved on Holi. Lovers apply colours to their beloved as an expression of their love and affection.
The legend of Radha and Krishna is enacted beautifully every year in places associated with Lord Krishna such as Nandgaon, Vrindavan and Barsana where devotees also take out processions in the honour of the Eternal Couple. The entire country gets drenched in the colours to celebrate the immortal love of Radha and Krishna. Thus, the whiff of love in the air makes this festival of Holi all the more joyful.