Bigen is a permanent hair coloring. Because it does not contain ammonia or hydrogen peroxide, it is not as harsh as regular hair dyes. It is effective for covering grays or damaged hair. However, Bigen can only make hair darker and can be difficult to remove once it has set in.
Once Bigen oxidizes, it can't really be removed so shampoo immediately after application if you are unhappy with the color. By shampooing out as much as possible right away, you may be able to lighten up the color.
Wait. Bigen only lasts four to six weeks, so any mistakes you've made by selecting too dark of a shade are temporary. The hair will lighten up day by day, and soon you will get the color you were originally going for. This is the best route for your hair, since you are still avoiding harsh chemical dyes.
If you can't wait, use a color-correcting product, like Colorfix. Because Bigen works to enhance your natural hair color, the results from a color-fixing product may not turn out exactly as you expect, but such a product will strip much of the Bigen from your hair. Usually, a chemical hair dye or a different shade of Bigen will be required afterwards to even out the color.