no artificial coloring doesn't really affect rats as far as i know...and the only down fall with using dog food is it can affect their hair...I have seen them loose just about all of their hair from being on a dog food diet. But yes it is still safe to feed your snakes.
They only way I would be concerned about what the rats/mice were eating is if they were being fed rat poison or were living in the wild. Dog food is fine for rats, although it doesn't always have everything mice/rats need to live long healthy lives, but that doesn't effect your snake. They also don't need to eat name brand or off brand, it really doesn't matter as long as they can live off of it. Cheap dog food has more grain, expensive dog food has more protein, so theoretically the cheap stuff is closer to the kind of food they should be eating.
Re: How concern should we be about our feeder rats diet?
I feed mine the higher protien rat and mouse block. It costs me 17 bucks a month and it helps during the winter when it gets cold in the garage. I know it probably doesn't matter but I just figure if I feed my feeders a better diet it will give my snakes more good stuff when they eat them. Like I say it probably doesn't matter but I do it just to do it.
Too many snakes to list. Ball pythons, Gtp's and Boa's. It easier to say it that way.