It depends on the type of ingredients you're using in the batter. Generally, the lentils help in getting the right color to the Dosas. You can also add some Methi Seeds (Fenugreek Seeds) for a darker shade of golden brown color. Apart from this, the final color also depends on how long you cook the Dosas on the pan. If you take them out quickly, your Dosas might have a white-light brown surface. But if you keep them a little longer on the pan, it will help in getting the golden brown color to it. It will also make them crispier in this process. Note: Don't keep them on the flame for too long, as they may get burned easily.