Temporary hair dye

If I dye my hair temporaily black does it always go black or does it sometimes go a weird colour like grey purple or blue and does it how long does it take to wash out and when you wash it out as its fading does it look greyish?

Depending on the color of your hair it is now,it could do many things.
As your hair grows,the color will fade over time.Its very likely that it will fade to purple,ect.
However if you intend on keeping the color in,then redying it every few months can help to reduce the fading and bleeding of other colors.
:]

if you're planning only to go black temporarily.. don't do it unless your hair is already very dark brown. I have a dark blonde/light brown hair and tried to do the whole temp black thing and it NEVER washed out. after calling the company, I had to have it professionally stripped and redyed to match my natural color. if the black is something you intend to keep, go for it. if not, I wouldn't recommend it.