I based this stitch map on photos of Nancy Marchant’s Leafy Lacey Brioche Keyhole Scarf. I have no idea if the stitch map matches Nancy’s pattern or not. Actually, it probably doesn't.
Note: Add in the selvedge stitches of your choice. Then alternate one row color A, one row color B.
Swatch photo courtesy Hannele Lievonen.
Shown with a cast-on count of 37 stitches.