Answer:
Blue is the colour of the Sky and the Sea - both of which are actually colourless. Blue signifies the vast expanse of space which pervades the universe and in which the Universe has its being.
VISHNU means the “all-pervading consciousness” and therefore his characteristic colour should be blue.
The connection between the Absolute known as BRAHMAN and expansive space is enunciated in the Chandogya Upanishad.
3:12:7-9: That which is (designated as) Brahman, even that is this ākāśa (space) outside the body. That which is the ākāśa outside the body, even that is the ākāśa inside the body. That which is the ākāśa inside the body, even that is this ākāśa within the (lotus of the) heart. This Brahmaan is all-filling and unchanging.
hope helpful <3