The food coloring is totally optional, but if you want to use it, the amount you should use will vary based on the brand you have and your personal preference. If you want to create the lime green color I have here, you will need both green and yellow. I used 5 drops yellow and 3 green, but again, your results may vary based on the brand you are using. I recommend starting with only a drop or two and adding more gradually until you achieve the look you are going for.