> Isn't purple expected to be there per ROYGBIV

I do not understand the Physics of the phenomenon very well, but I would expect the high frequency wavelengths (for example, purple) to be scattered away.

"Surround" does not describe the phenomenon of Belt of Venus adequately. Think of it like a horizontal band wrapped around the dome of the sky, slightly above the horizon, and prominent towards the West, fading towards the East.