I raise full size mules, and it depends on the color of the jack, what his colors are in his background, and what the mare has. If you breed to a red roan jack, the gene color is the most dilute. The theory is the color of the mare will over ride the color of the jack and come out on the off spring. I bought a red roan for that reason. I'm trying to breed and raise appy mules. The one on the left is my mini spotted mule. He's out of a red roan jack and a spotted Shetland mare, or really tall mini mare. He's 39†The other thing to know about breeding donkeys is your off spring will almost always be taller then the jack. Many don't worry about the size of the jack, because the off spring will be at least the size of the dam, if not taller.
My guess to get a colored mule out of your mare, I would breed to a mini red roan jack, and make sure he is red roan for a few generations on his papers. Roan Jacks aren’t like horses, they don’t pass on the “roan†factor. You won’t have the worry of if it’s going to be roan. The difference between a red roan jack and a sorrel jack, truly is color. Sorrel jacks will produce almost 80% sorrel, at least from what I’ve heard. Sorrel look like a sorrel horse, and the red do look more red, but will start to roan out as they get older, so they almost look pink. Many people try to pass off their sorrel jacks as red, but check your colors before you breed to them. Make sure you are breeding to the color you want.
Best of luck.