Dun genetics are always confusing. Grulla/grullo gets used as a "blanket" color way too often. It isn't really black or grey or this or that, so it must be a grulla. Or it has a dorsal so it must be a grulla. There are some good tips and tricks you can follow to help determine for the most part, but for every rule to a horse color, is an exception because the number one rule is the rules don't always apply! LOL. Duns must come from at least one dun parent so that can rule a lot of things out, if you have a grulla looking foal out of a double cream and bay mare...more than likely it is a smoky black...this is just an example. Another rule is duns must have dun factor BUT that is one rule that is not always set in stone...because a lot of other genes can affect the
appearance of dun factor. It
is true that all duns DO have leg bars etc regardless of what has been said, the difference is you can't always see them, but genetically they are there. The sooty gene, cream gene, grey gene and roan gene can all affect the appearance of dun factor. Some grullas/duns have black come up so high on the leg that it merges with the leg bars and the leg bars "seem" to not be there. Some horses with the "wild bay" coat pattern have black so LOW on the legs that again, it seems as if they do not have dark leg barring. So you can not always go on the APPEARANCE of dun factor but they do always carry it if the dun gene is present, but it can be masked. I have also noticed that minis are really good at not showing much leg bar, I personally think it is because the leg is shorter and doesn't leave much room for all the color candy that a longer leg does. Look for dorsals that go down into the tail, dorsals that seem to stop flat right at the edge of the rump/beginning of tail are usually counter shade. Sooty is very good at mimicking dun factor and so is champagne. Many champagnes in the larger breeds especially, have very good false dun factor. I have seen sooty buckskins with very convincing dun factor but usually there is just something about it that doesn't quite say "dun gene" if you know what to look for. I have a dunskin Missouri Foxtrotter. She tested Dd. She does not have an ounce of dun factor. Nada. She has the wild bay coloring and black very low on her legs. She has NO dorsal, ear bars, shoulder bars or leg bars...no zippers, nothing. She has a very frosted mane but that can be found in buckskins too. She was out of a red dun mare and smoky black. I tested her because as a foal she did seem to have dun characteristics and her coat color does seem more "dun" (a flatter tone) than buckskin. There is a slight period when she sheds from winter to summer coat where you can see the most faint dorsal, shoulder and leg bars. The dun factor is there, it is masked in her case. I was actually surprised she tested dun. So for sure, when in doubt test. I have known a few others that had no dun factor present at all, test D... The dun gene is still not 100% understood but for sure, if you are going to advertise a "rare grulla"...that doesn't drip with dun factor and has a dorsal only in a certain season etc, etc, I would like to personally see the claim backed up with a test.
I love duns, they are the best color in the world, some think all horses carry dun factor but unless it is acted upon by the dun gene (or others) it is hidden. This may be true since some genes also seem to bring out "dun factor appearances" like sooty etc. It has been my experience in the mini world (not so much in other breeds I am involved with) that most minis advertised as grullas OR duns are silver dapples or smoky blacks or sooty buckskins...just my experience and I am ALWAYS on the look out for a dun.