Although it seemed like a good idea at the time, your dyed mustache may have turned out just a hair too dark. If that's the case, you can use hydrogen peroxide to lighten the shade gradually. Hydrogen peroxide is a cheap, mild and effective bleaching agent that can correct this issue in a few minutes.
Make a 50/50 mixture of the peroxide and water.
Dampen your mustache by rubbing wet fingers through the mustache hair.
Dip your mustache comb in the hydrogen peroxide solution. Using a diluted 3 percent mixture allows you to add the compound in small enough concentration to avoid accidentally over-bleaching, while still giving you the lightening effect you're seeking.
Comb the hydrogen peroxide solution through your mustache. Make sure you get even coverage. Combing is preferred over spraying -- as you would with applying peroxide solutions to the hair on your head -- due to its more targeted approach. Plus, you do not want to accidentally spray it in your eyes.
Let the solution sit for 10 minutes. If you have to significantly lighten the mustache, you can let it sit longer. Do not allow the solution to remain on your mustache for more than 30 minutes, since longer than that will potentially lighten the color too much -- and lighten your lip as well.
Wash your face to remove any residual hydrogen peroxide.
Apply your normal hair conditioner to your mustache to offset the drying effects of hydrogen peroxide. Leave it in for a few minutes, then rinse your mustache clean.