I've rarely met an old art school color theory exercise I didn't like, but this one is other level, feeling like a finished work that veers into Op Art territory.  Dated 1973 on reverse and signed by a Sally Winter, it was executed on a large sheet of gridded paper in what appears to be gouache though perhaps some combination of paint and marker. (Details give a good sense of both color and surface.) Very saturated color, very well done, quite striking. I found it with a mat around it and inside a mylar sleeve, so it has been very well preserved. I am happy to include the mat, which is bright white, if you like; just let me know.

15" x 15" and in very good condition, with perimeter taping on reverse which makes it pretty sturdy.