What we now call the international Buddhist flag was created in the late nineteenth century. It represents the six colours of the Buddha’s aura, the light that could radiate from his body when he chose. The sixth color is called pabbhassara. This is often considered to be a combination of the other five. Sometimes it is translated as pure radiance.
We find a mention of the colours in the Buddha’s aura in the story of the miracles he performed in the town of Uruvela, shortly after his enlightenment. (Although the list of colours is slightly different.) The following is the version of the story in verse (Angirasa is another name for the Buddha):
Near the Nerañjarā, the Lord
spoke thus to the matted hair ascetic Uruvelākassapa:
“If it is not inconvenient to you, Kassapa,
let me stay this day (only) in the fire-hall.”
“It is not inconvenient to me, great recluse,
(but) as I am anxious for your comfort I warn you
that there is a fierce serpent king there,
of psychic power, a terribly venomous snake.
Do not let him harm you.”
“It is not likely that he can harm me.
Please do you, Kassapa, allow (me the use of) the fire-room.”
“It is given”; having understood this,
the fearless one entered, fear overpassed.
Having seen that the holy man had entered,
the chief of snakes, afflicted, blew forth smoke.
The chief of men, joyful, unperturbed,
blew forth smoke there too.
But the chief of snakes, not conquering anger,
blazed up like a fire.
The chief of men, highly proficient in the condition of heat,
blazed up there too.
When both were in flames,
the matted hair ascetics, as they were looking at the fire-room, said:
“Beautiful indeed is the great recluse,
(but) he will be harmed by the serpent.”
Then at the end of that night
the serpent’s flames became extinguished,
but the multicoloured flames of him of psychic power remained,
and multicoloured flames, dark green,
then red, crimson, yellow and crystal-coloured
were on Angirasa’s body.
Having put the chief of snakes into his bowl,
he showed him to the brahmin, saying:
“This, Kassapa, is your serpent,
his heat was mastered by heat.”
Mahakhanda, Vinaya Pitaka