It was designed in 1978 and originally had 8 colours:
Color Meaning
Hot Pink: Sexuality
Red: Life
Orange: Healing
Yellow: Sunlight
Green: Nature
Turquoise: Magic/art
Indigo/blue: Serenity/harmony
Violet: Spirit
Gilbert Baker (the man who designed it) was said to be inspired by Judy Garland's Somewhere Over The Rainbow, and/or the hippie movement with their 60's "Flag of All Races"