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.
The color of Vishnu is the color of a dark blue cloud. It is the color of the sky, denoting his cosmic dimensions and his connection with the Vedic gods of rain and thunder and his relationship with the earth.
According to symbolism, Blue colour represents that He is Infinite like sky & ocean.
Vishnu's blue body is often depicted in blue. The color blue symbolizes the infinite. Vishnu is represented as an infinite force. He has no form, no name, and he is incommensurable.
He is Bluish because he is infinite and immeasurable like the sky which appears Blue .
Vishnu is depicted with a blue skin. The colour blue symbolises the infinite expansion of the blue sky and the blue ocean.
Besides, Vainkuntha is the eternal home/abode of supreme lord Vishnu. The glimpse of Vainkuntha is described in Bhagavat Puran as follows:
(According to the wikipedia, At the time of the creation of the cosmos, when Brahma satisfied Lord Vishnu with penance after being born on the lotus emanating from the navel of lord Vishnu, Lord Brahma was shown a glimpse of this eternal abode/home of Vishnu.)
.....The inhabitants of the Vaikuntha planets are described as having a glowing sky-bluish complexion.....
.....and all these combined together appear just like the sky decorated with both clouds and lightning
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.