I used dark brown permanent hair dye on top of my reddish colour so that I can try to put blond on like I used to have but it came out too dark, any idea how I can fade it out back to my natural mousy brown colour? Or can I not?

A hairdresser can do it, or you can now get products which are designed to break down hair dye and leave you your natural colour. Ask your hair dresser or a good hair product place.

First, I'd like to say (as a hairstylist) that until you know the fundamentals of hair color, you should probably have a professional color your hair (or fix mishaps). With that said, my suggestion to help this dark color fade faster is this: Dish soap. Use a hand dish soap such as Palmolive as a shampoo for a week or so. Shampoo no more than twice a day with it, though, or your hair will be stripped of too many natural oils it needs to stay healthy. After a few days to a week, you will want to give it a deep conditioning. If you don't have the opportunity to go to a professional hairstylist for the conditioning, you can use a regular moisturizing conditioner. Put on your dry or damp hair generously and cover with plastic shower cap and wrap with a hot, damp towel for 20 minutes and then rinse. Or you can use an at-home recipe of 1 cup mayo (room temperature), 1/2 cup olive oil, and 3 egg yolks. Follow with plastic cap and towel and sit 20 minutes before rinsing. As for the color, there is a chance it may not fade completely because it did go so dark. The rest may just have to grow out unless you can pay to have it fixed. Do NOT try to continue to color it to fix it. The rule of thumb is that "color on top of color will always go darker". Having some highlights put in to blend the natural color and the dye while it grows out is an option, too.