I think the early depictions of the halo in christian icons were just plain gold spheres.
With the exception of Jesus. His halo was usually shown with a paler cross inlaid into the sphere.
There aren't many authentic early depictions around because the idolic nature of them caused the iconoclastic schism, many were destroyed. The radiating lines and alternate colours were added much, much, later and are entirely down to artistic interpretation of the cross motif (I've often seen radiating lines between the arms of a cross, as well as, around the 'holy heart').