I don't think it is a silk bandeau because on the zoomed-in picture I can spot some framework of a diadem. Maybe it is the diadem from the Emerald Parure but with the central prong replaced by a small brooch? That is also a nice option for Queen Máxima or Princess Laurentien by the way.
I think Queen Juliana wore the bandeau with the old rose-cut diamonds, the position of the camera (low and frontal) takes the sight away on the bandeau (the Queen looking upright and her hair in front of the diamonds).