I used brown hair color and it turned hair red! What to do to fix?

If your natural color is dark, then what you see is very common. You can either color it with a darker dye (ash brown, for example; something without red in it) or you can use color remover (ColorFix; Color Oops) to get out the dye you just used (if it's a dark dye) and then bleach it past the red stage.