Hey y'all! I was wondering why my breeding stock paint mare produced a bsp when bred to a b&w tobiano stallion?

My mare is a registered apha bsp. Dark bay with an almost all white face and white eyelashes. The sire is a black and white tobiano. She just had her foal and from what we could see it's either a brown or bay with a star and snip, one tall stocking and a small sock on her hind legs. I thought that we'd have some kind of paint since the sire was a tobiano bred to our bsp. Why did she turn out to be a breeding stock paint instead of a colored paint????