There is a genetic test available to differentiate between the agouti bay versus agouti brown allele. It's not very expensive if you would like to know her genetic color identity. There are some horses that phenotypically do not truly exhibit their genetic color. I have 3 mares that no one would accurately identify their genetic color just by their appearance.
Looks like a horse at the ranch, and she is classified as a dark brown bay. So ill say that! The lighten on her flank and her face but dark legs and dark mane and tail is what makes her a dark brown bay.