Chances are she has more than one pattern going on. She is a loud frame overo. See the black going all the way down her back and into her tail? An overo will typically have the dark color covering the whole back.
Chances are she carries the sabino gene as well, but APHA doesn't register the sabino pattern. It's either overo, tovero, tobiano or breeding stock. She could also carry the splash gene too which would account for the excessive white on her legs and face.
Tovero would be a cross of dark color and white. Tobiano can do that as well.
If you really want to know, get her tested. If you ever breed her, get her tested for Lethal White Overo Syndrome before exposing her to a stallion. It's only $25 bucks and the results are back in just a few weeks. APHA.Com - Overo Pattern