"Is there something safe with more color to add to the tank that would float on top or mix in so that I could see the level?" That is an interesting solution to your issue. Could almost be borderline brilliant actually. They do make fuel dye for tinting fuel. Certain food colorings are petroleum based too. 2 cycle oil may be a temporary thought to test your theory. You wouldn't want to use it all the time as it would create too many extra emissions and engine deposits.