Will it be OK to put another colour on , a light brown one , tomorrow

Well you may take a chance on your hair being damaged. It all depends on the condition of your hair now. However if you put light brown over dark brown it usually will not lighten it - sometimes it will. If you are not liking the color your dark brown turned out to be, I suggest you go buy Color Oops. It will not strip your hair like most other products do. What it will do is take you back to your natural color. You can buy it at most drug stores. Here is the companies website Developlus - Color Oops Here is a video about it YouTube - Beauty Truth Test: Color Oops hair color remover