So, Gilbert Baker was the first person to make the rainbow flag in 1978. The eight colors (which is now six, I believe) used were meant to represent different parts of people coming together.
Hot pink: Sex
Red: Life
Orange: Healing
Yellow: Sunlight
Green: Nature
Turquoise: Art/Magic
Indigo: Harmony/Peace
Violet: Spirit
One of the main reasons for its creation is that no one wanted to use the pink triangle which had been the closest thing to a symbol that they had (obviously it had bad backgrounds, due to the use by Nazis as an identifying symbol)