Humans (at least Asians) are wheatish-brown in color. Why was Krishna portrayed as blue? Is it just to show his purity, or is there some story background of it (e.g. is it based in reality)?
Krishna's color is quoted from scripture to be like the hue of newly formed rain clouds. See Brahma Samhita 5.30:
veṇuṁ kvaṇantam aravinda-dalāyatākṣam-
govindam ādi-puruṣaṁ tam ahaṁ bhajāmi
For the full word for word translation, see Bhaktivedanta VedaBase. An overall translation is below:
I worship Govinda, the primeval Lord, who is adept in playing on His flute, with blooming eyes like lotus petals with head decked with peacock's feather, with the figure of beauty tinged with the hue of blue clouds, and His unique loveliness charming millions of Cupids.
Krishna is not the only avatar/deity to be represented as a blue figure. He's accompanied in this by Rama and Kali to name just two.
The blue color is a symbol. Both Rama and Krishna are considered to be shyama varna aka of 'dark color'. According to Amara-kosa: trisu shyamau harit-krishnau - the word shyama means harit (“green”) or krishna (“black or dark blue”).
So, in terms of 'actual skin color', these folks were probably supposed to be very dark skinned (also something very very possible in the Indian subcontinent - possibly even more so than the wheatish complexion), with Krishna possibly supposed to have a blue undertone to skin.
After all, the Jagganath representation of Krishna is portrayed as 'black' not 'blue'.
To be extremely mundane, possibly, blue came to be used as a symbol because it's easier to portray blue skin in visual representations than really dark / black skin.
Excerpts from a talk by Sri Sri Ravi Shankar:
See Krishna is always depicted in blue colour. You know why in blue colour? It means that the body is transparent as if it was not there. He was not a blue baby, certainly! Even Shiva is depicted in blue. That doesn’t concern the physical body but inside is infinity. Whatever is infinity is represented by blue: sky is blue, ocean is blue, the great, the big, enormous depth and that bliss, that being- the soul. The soul is never born, but still it is born when the mind and the prana and the body come together.
The same question was asked by a saint in a school to kids. No body was able to answer except one.
That kid said, 'All infinite and limitless things are blue, sky is limitless is blue, the sea whose depth is limitless is blue, similarly God is limitless and infinite and thus is shown blue too'. The saint was pleased with the answer.
Well the name of the saint was Swami Vivekananda and the kid who answered was Lal Bahaddur Shastri ji!
I came across the above story in a marathi book named Jeevan Geeta by author Ram Keshav Ranade. He was a judge and was a famous Krishna devotee in the state of Maharashtra. Krishna used to come in his dream and used to bestow him with His grace.
I personally like this reasoning of Lal Bahaddur Shastriji.