Hi Janina,
Try using a clarifying shampoo once per week, such as the Sunday Shampoo by Bumble and bumble. On regular basis, use Bumble's color support shampoo for cool blondes. Regular brushing with a natural bristle brush will greatly help as well. If you would like for us to send you these products, please call the salon at 212-677-7315 x 4. Finally, you can ask your colorist to do a clear glaze about every six weeks, which is like a clear top coat for your nails, and will add nice shine to your hair if properly applied. Hope this helps.
Best
Eva