Check your first aid kit or bathroom vanity for ordinary 3-percent hydrogen peroxide, which is the equivalent of 10-volume cosmetic grade hydrogen peroxide. No need to panic if you don't have it -- your drugstore or supermarket will have it (you just may need to wear a hat while you go get it).
Mix equal parts of peroxide and shampoo in a plastic cup and mix to combine with a cotton swab. Spray your hair with a spray bottle to dampen it. Don't saturate it; just give it a few spritzes to moisten the ends.
Scoop out the shampoo and peroxide mixture (soap cap) with your fingers and massage it into your hair, focusing mainly on the ends. Don't scrub it into your scalp, that's not the problem area. You will end up irritating your skin.
Leave the soap cap on your hair for three to five minutes, checking continuously in the mirror to see if the purple is dissolving. Remove the soap cap as soon as the purple is gone.
Rinse your hair with warm water when it is time to remove the soap cap. Condition your hair afterward to reintroduce some moisture to your hair, then rinse, dry and style as usual.
If you do not have a spray bottle, splash some water on your hair (like you are washing your face) and comb it through to your ends with a wide-toothed comb.
Keep in mind that discolorations of this type are most often a porosity issue. Therefore, do not use a peroxide any stronger than 3 percent. If you do, you can end up with breakage, severely damaged hair and even further color issues.
Remove the soap cap immediately if your scalp begins to burn. Rinse your scalp with cool water for 10 to 15 minutes. If the burning sensation does not resolve, seek medical attention.