Annette,
I recently went to the Desert Museum in Tucson, which has a walk-in hummingbird aviary and has done extensive hummingbird studies, and they said never to add food coloring to hummingbird food. This is supported by the Audobon Society. Here is a line from the Audobon Society, "Fill the feeders with sugar water, made by combining four parts hot water to one part white sugar, boiled for one to two minutes. NEVER use honey, which promotes the growth of harmful bacteria, or artificial sweeteners. Also avoid red food coloring". If you google red food coloring in hummingbird feeder you will find that it is a very hot subject but that nearly everyone agrees that it is best not to take a chance on the possible harm the food coloring might cause. It is also important to make sure the mixture is not diluted to less than 1 part sugar to four parts water. I know you love your hummers and would not want to take any chances on harming them.
Casey