I first encountered this idea of "Indigo" as an aura color in 1997/1998 when reading what is a personality theory book by Barbara Bowers, "What Color is your aura: Personality spectrums for understanding and growth". Being the _N_P that I am, I felt no need to belief in either auras or reincarnation in order to see value in what was being said. Please note that the author developed the content of this book based upon her observations while reading the auras of others, and yet, to me, almost none of the book required any "belief" in auras to be meaningful/useful. I think the Indigo section touched a bit on spiritual/psychic stuff, and I think the crystal section did more. But otherwise, I don't recall any of that psychic/spiritual/aura stuff in the descriptions of the other colors/personalities.
I scored very highly for Indigo, and quite highly for crystal, though at other [admittedly much rougher/assetive/physical] times in my life my second color was,as I recall, physical tan. As I recall from her first book, Pamela Oslie trained under Barbara Bowers.
To me, there nothing in Bower's book seemed to "New Agey" or "out there", but I also haven't read it in a while. IIRC, I thought the crystal description was more out there than the Indigo one. Then came Pamela Oslie's book, which to me had a "Indigo's are so psychically advanced" tone, which I did not care for at all. Then came Tober or whoever with that "Indigo children" book, which seemed to say things like "Indigo children don't feel a need to talk to adults because they are too busy telepathically communicating with other children!", at which point I totally had a WTF reaction!
I think Oslie's second book, or at least the second one that I know of, Love Colors, toned down the psychic/New Age greatness of Indigos to a point that I could tolerate and live with, it was her first book that bothered me. Please note that I liked both "What Color is Your Aura" and "Love Colors" enough that I bought them, and they are both in front of me write right now while I am typing this. The "Indigo Children" books that I have read offended me so much that I wanted to spit on them or something, and I don't spit on books!
FWIW, I have a lot of background in eastern philosophy, yoga, and martial arts. I know and have known many people that would describe themselves as "psychic." I personally think that many things exist within the realm of human potential, but "little special Johnny is going to save the world because of his special indigo telepathicness", it makes me wanna get a zen stick and smack people around for their self-deluded idiotness. Also, in one of the "Indigo children" books, there was an interview section, and some like 14 year old was saying things like "I feel so different from others, and appalled by our consumeristic society, and like an outcast AND I JUST KNOW THAT I AM ENLIGTENED, I CAN JUST *FEEL* IT" [bolding mine]. I "believe" in enlightenment, I believe that there are enlightened individuals in this world, but if you look at say Zen Buddhism, there are standard tests that they do to make sure such an experience is actually happening and correct, and not just wishful projection or taking something that isn't enlightenment and thinking that it is enlightenment. If this like 14 year old is so darn confident in their attainment, let some, say, Zen master have a take and them and confirm it. I'm pretty darn sure this like 14yr old is no such thing as enlightened, but if for some reason they are then let them take their place alongside the likes of Hakuin, Bodhidharma, the Buddha, Lao Tzu, HH the Dalai Lama, Kalu Rinpoche, Ramana Maharshi, Nissargadatta Maharaj, etc. IF said 14 yr old actually was enlightened, I think they'd be doing something else with their life other than, apparently, lying around and whining about how others don't understand or accept them.
Not that I have any strong feelings or reactions on this matter now or anything....