What do you think is the best age to dye my hair? And what colours will be good for long hair?

I don't know about specific age, but I think the best time to do something like dying hair is when you can make a mature and concise decision. 

So maybe when you're a bit older. The usual consensus on best dye colour for long hair is usally the primary hair colours e.g. ginger, brown, blond and black... but then again, best colour is a very subjective opinion, so go with what you think is best.

Personally if I was getting my hair dyed I would want rainbow colour just to be like one of the best characters from mlp: rainbow dash.

I'd have to say maybe 16, and it all really depends on what you like. I have long curly brown hair, when I'm older I wanna dye it a deeper shade of brown, because I look better with darker colors. :)