Bleaching the color out of hair involves using a strong base that also causes damage to the hair. The actual damage done by the bleaching agent helps to remove THC metabolites as well as making the hair slightly porous which may help remove even more metabolites over the next few days. Dying the hair back isn't to help with the test but so that your boss doesn't know that you went Billy Idol to beat the test.

I know people who have used Zydot and passed and I have also seen a government studt that showed Zydot can help reduce THC metabolites in the hair by up to 39%.

Basically, the more you smoke, the more you need to do to your hair to remove the metabolites. It is hard to do and doesn't always work but it can be done.