A classic case of colour-lift caused by frequent shampooing with the ` wrong` shampoo ...you need sulfate-free shampoo or one specifically formulated for system-hair....such ` treatment` will only exacerbate normal / natural oxidation caused by sun etc.
You can rectify it by using a colouring product... you can use this link to find your shade:
http://www.justformen.com/products/find_shade.asp
Don`t leave this on/in for longer than about 4 minutes max. [apply it carefully to damp hair--keep it off the lace] When the hair is dry, check the ` depth` of colour..if it`s not dark enough, wet the hair again and re-apply .... only leave it on for 2 minutes or so.....you don`t want it to go too dark, or you`ll have another problem. System hair takes colour very rapidly compared to growing hair so ignore the instructions on the pack...proceed as I have outlined. Apply it carefully with a comb which has some of the ` mix` on the spine/teeth....it`s not difficult...just watch your timings.