My favorite color is the Grulla. If I have a buckskin mare, what is the probability she would throw a grulla? Would it be best to breed to another buckskin, or a grulla. I have no clue but I deffinatly want to breed her to the best possibility.
I would say your chances would be slim, even if you bred to a grulla.
Being a buckskin, your mare is for sure carrying one black gene (possibly 2) agouti and cream.
To be a grulla they must have atleast one black gene and dun. No agouti.
If you bred to a grulla there would still be a good chance that you would not get a grulla but probably a bay dun or a dunskin.
Your best bet would, as CLaPorte423 said, would be to buy one.
If the mare is homozygous for agouti (carries two copies of it) there is 0% chance of her producing a grullo foal, no matter what color the stud is. As a buckskin, she's guaranteed to carry at least one copy of it.