Actually, there are 13 chakras (not mentioning the nadis, meridians, 16 secondary chakras, 14 plexus (plexuses? Plexi? :p) and 5 internal minor chakras), to which you can add 3 subterranean chakras. The eighth chakra is not the aura, it is just another chakra located roughly 50 cm above your head. No colour associated that I know of. Its main purpose is to override the mind to connect and resynchronise with the void/matrix/whatever you call it and is therefore quite good for meditation.
All humans have 16 main chakras, the 7 body chakras are always activated, the 8th and 9th and their subterranean counterpart (-1 and -2) are often initialiazed and can be activated but not always (although there is little doubt if you're practicing psychic stuff/energy/spiritual things), the 11th (plus -3 as its grounding counterpart) and the 12th are more rarely initialized (but they can be even without knowing it if you pratice a lot of meditation/yoga/qi gong or received some kind of initiation), and the 13th is a bit special: you have to do a special ritual with someone of the opposite sex having his/her 12th chakra initialized, and face to face, you both visualize your first chakras fusing into one, then your second, etc., until the 12th, and then the 13th should initialize itself for both. From that moment on, both persons will be able to activatr their 13th chakra whenever needed. Allegedly, some sacred sites are designed to activate the 12th or 13th chakra, but I don't know more about that.
