The problem is you don't know what the natural colour of your hair is (or rather the natural mix of colours) until you let it grow out for at least a couple of weeks. But you can't let it grow out for a couple of weeks, because then there is an obvious line of demarcation between your real hair and your wig.

My suggestion is to get a new blond hairpiece and put it to one side. Then take a 3 week long vacation on which you don't wear your usual piece. Wear a ballcap instead, and let your hair grow out. At the end of the vacation get a short haircut on your real fringe hair, cutting off all the dyed ends. Then buy some hairdye and dye the blond hairpiece to match your real (grey) haircolour. In a few weeks you will have grown enough natural grey hair to get a new Toplace piece in your natural colour.