How do a lighten a dark hair dye

If the hair hasn't already been dyed, and you're only trying to lighten the shade of the dye, you can try mixing it with a lighter shade of the same brand, but you may have difficulty predicting the final color.

If the hair is already dyed, you need to use a color remover like ColorFix, Color Zap, Color Oops, or Igora Phantom to remove the artificial pigment from the hair. You then have to re-dye the hair the lighter color.

Remember also that single-process haircoloring is limited in its ability to lighten. You can't pick just any single-process haircoloring and expect to get the color on the package.