Combine water and sugar in a small saucepan, bring to a boil and simmer over medium heat, about 15 minutes. Remove from fire and cool.
Combine sugar syrup, coloring (if using), lemon juice and lemon zest. Pour the mixture into a container and refrigerate for at least 4 hours. Every 30 minutes. Take the container out of the refrigerator and stir the mixture with a whisk.