so everyones explained grey pretty much, it's just the grey area of magic. like cast a healing spell on a friend. you're doing it to help, that's good, but you didn't ask and therefore you're imposing your will on another. this is what grey magic means, you need to think about it, it's not good or evil [white or black]
2. hard to describe, i'll come up with an example dream.
in the dream you are walking down oak street, and you come across a brown kitten with one brown eye and one blue and you take it home.
a week later, you're walking down maple street and find a brown kitten with brown and blue eyes. you dreamed about the future, just one certain thing. if you were on the right street, then it's a vision. it's pretty tricky since there's also interpretation of dreams, because you could say you were dreaming about a street you know, or the dream might be telling you to interpret 'oak' along with 'brown' 'kitten' 'eyes' 'blue' add that together, and it's telling you about something. that's where your question can get confusing and complicated. personally, i don't see why it's important to know the difference, either way you 'saw' the future.
3. there are so many types it's hard to list them all. while magic has no colour it can be categorized into 'black' [negative magic like hexes] 'white' [positive like purification] 'grey' [love and healing] and green. green isn't really the magics colour, it's for practitioners of magic centered around herbs, natural remedies, and infusing magic into everyday life. reiki, candle magic, dragon, poppet, faerie, potion [any magic where it involves liquid; tea, oil, baths ect] i know there's more, but i can't think of any currently.