Sort of. I'm about to go to bed, so I hope this makes sense. It makes sense in my head, but relaying it right now might be fuzzy. Any color that is strictly black + a modification other than agouti (champagne, cream, dun, silver, etc) cannot have agouti because that would cause a different color, which takes out a large number of black based horses.
While not entirely correct to say because bay is black-based, it would be easier to think of it as any bay-based horse could also be carrying At. A horse can be AAt, but it would still be a bay horse because A is dominant over At. And it is also easier to say that any red-based horse could be carrying At. At least with a black-based horse, we can make assumptions based on its phenotype on what agouti mutation it may carry, but red based horses have no black to restrict, leaving zero possibilities to determine agouti status without a DNA test.
Abby's best friend was a sorrel Appy with snowflakes along with their gray friend. She's buddied up with Kaja now, who is a dun. Coincidentally with your post, the gelding they live with is a palomino. Where I board my horses, there are 3-4 horses per paddock who are grouped based on gender and personality. We don't have much variety in color anyway, so there really aren't patterns.
Last edited by Poseidon; 03-26-2013 at 12:30 AM.