...and that's the main reason blocks are better than mixes. Rodents will always pick out the treats first and eat the nutritious stuff last. My rodents don't have stools that loose unless they are sick and it's never that color because there are no added dyes in their food.
I know that a lot of these animals are bred as feeders and that pets stores don't particularly care about the health of their animals, but I still think they deserve adequate nutrition. AFRMA (the American Fancy Rat and Mouse Association) recommends lab lock or high quality dog food as a staple for a reason. It's better for them. Poop like that is only normal if they're eating food that contains a lot of unnecessary supplements (to make up for the nutrition it lacks) and food coloring (to make it look nice for the humans who will buy it). It's not good for the animals.
Kaytee in particular is known in fancy rat/mouse circles for making awful food.