Why is Lord Vishnu portrayed in blue color? Is there any incident in Hindu mythology explaining this? If yes, then please do share that story!
Adding to Pandya's Answer, Skanda Purana explains how Vaikunta became blue and gives a reason for His color.
The chapter 9 Churning of Ocean in Skanda Purana says Halahal Poison made Vaikunta blue and Lord Vishnu to color of Taamala.
- By that time, the great poison, Kaalakuta came there. After burning Brahma's world at the outset, it burned Vaikunta.
- Vishnu who dwells in the cavity of the heart of everyone, was burned by the fire of Kaalakuta also with his attendants. Immediately he acquired the color of Tamaala.
- Vaikunta also became blue in color. It was surrounded by all the worlds. Hence all the worlds became encircled by the poisonous substance from the waters.
Tamaala is Indian bay leaf. I checked Sanskrit Verses and word Taamala is used to describe Vishnu. Bay leaves are generally olive green in color but dried leaves are brown in color. So, i'm not sure about what exactly does author mean by color of Tamaala.
Vishnu's blue body is often depicted in blue. The color blue symbolizes the infinite. Vishnu wears yellow clothing. The yellow color is associated with the terrestrial life.
Color of lord Vishnu is yellow. That is why he is called as pitambar.