can you fade light hair to natural color if its dark?

I don't know for sure, okay? But the dye you put in your hair to make it lighter had to have some bleaching effect for it to work from your very dark hair. If you used dawn, you would probably just remove the ash tones in your dye job (you said your hair was now light ash brown?) and wind up with a slightly lighter, less natural looking color.

If I'm wrong, do correct me... but it seems like the easiest thing to do here would be if you want to go darker again, just have it dyed darker.