How common is dun color and dorsal stripe in a Quarter Horse?

I’ve been horse sitting my sister-in-law’s beautiful quarter horse mare. I’ve researched her color and markings and found out they are "primitive". She has faint stripes on her shoulders and dark legs, tail, and mane and a white-ish star on her forehead. Is this common or rare in a quarter horse?

It depends on what area of the country you are in. Some areas, such as South Texas, dunns are fairly common. A buckskin, dun, cramello, gulla, palamino…ect are in the buckskin catagory of color, caused by a deluding of the black gene, and are more common in the southwest than many other part of the country. Much like if go up around Oregon or Washington, Appaloosa will be more common due to them originating there. Now a zebra stripe buckskin or dun are more rare than a buck, or dun with out them. Good luck