It is common for tinted red hair to fade fairly easily (usually because permanent hair tints do not have a great deal of color pigment in them). Want to know a secret about keeping red hair red? If you want to maintain a nice color in between tints, use a red colored shampoo.
Aura makes red colored shampoo called Madder Root Shampoo (companies like Wella, Loreal, Clairol and others also make shampoos that are made expressly for use on color treated hair).
*These shampoos are mild and will not strip out color.