Hi Arabella
I'm not sure if the following will answer your question but I have just read a piece in the latest Hair magazine which discusses the reason red hair fades more quickly than other shades. Apparently "Red pigment is generally more difficult to get into the inner structure of the hair, and if your hair is particularly porous or weakened, the red colour pigment tends to fade more quickly too." The mag. recommends either having a professional conditioning treatment before you apply your colour and to use colour-saving shampoos and conditioners to help maintain your shade.
Hope this helps a little!
Fi