In my "introduction to spirituality course" we were given green triangle drawn on paper to look/watch/stare at and see what will happen.
I had it completely become invisible from the paper(no not erase but me seeing blank paper, no triangle there when I was concentrated enough), then had what felt like when clouds cover the sun and suddenly surroundings, peripheral field of vision become darker.
Next month we were to draw yellow one (and given explanation) that yellow triangle "enhances spirituality" (I know explanation sucks but can't remember and explain it better.)
And if it was red (triangle) it would be fire.
Dunno if it helped anyhow but that's all I "know".