I am not a scientist. I don't have a well-stocked, state-of-the-art laboratory where I can do research with controls and data and irrefutable results. What I am is a dog lover, owner, trainer, and handler. What I have is life experience.
What I have observed is the unmistakable difference I see in dogs when they eat a dog food without artificial colorings and preservatives. I have observed dogs appearing much more comfortable in their own skin when they are fed a food without a lot of corn in it. I have seen remarkable positive changes in a dog's focus and work ethic without all the junk in their diet.
Feeding a quality food to your dog does not have to cost any more than feeding your dog food full of colors and preservatives and grains. In fact, when you switch to a better quality food, you can feed less because there is less filler and more substance to the food. As a result, you don't go through food or money as quickly.
My theory is that dogs get addicted to the artificial colors, the preservatives, and the sugars (corn is basically sugar). People tell me they can't get their dog to eat anything else. If you were addicted to something, you wouldn't want anything else either -- is that what's good for you though? We have switched several dogs to different foods. The results have been phenomenal. One dog almost immediately began calming down, focusing more, and learning at an accelerated rate. In another dog we saw significantly reduced levels of reactivity and aggression.
Dog foods with color and a lot of "bells and whistles" are made to attract the human consumer. Not necessarily for the good of the dogs. Be a discriminating consumer. Eliminate the dyes, the chemicals, the "bells and whistles" and you will have a healthier, happier dog.