My factory highlights have been perfect when I gave them exact specs.

The real issue I have with adding them later is the potential damage they can cause to the hair. If you bleach the hair in ANY way you are essentially bleaching hair that has already been stripped of all color and then re-colored once.

Then again, I have bleached a whole piece slightly, but never done highlights only. The hair that I bleached (with a shampoo buffer) shed like crazy. That's why I would try to avoid it.

Proceed with caution....