A little dye mishap.

Well, A few weeks ago I redyed my hair black. Assuming all the color washout had run it's course, yesterday I bleached my tips in order to dye them blue. When I washed out the bleach the black bled into the newly acquired blonde and turned the majority of my ends gray.

Now I am not new to hair dye by any means, but why did this happen? And how can I prevent it from happening again?

Thank you all!

Always rinse with as cold of water as you can stand, it helps keep the cuticle closed thus preventing some bleeding. It's usually too much work for people, but you can also cover the black in a thick conditioner [like cholesterol] and wash it separately from the tips, and vice versa.