They just grabbed a card back and it happened to be the wrong one. CFB has already stated that it isn't the right back of the card.
Ok, the card back for that Sol Ring was an error, but isn't the existence itself of that back proving that WotC have plans for an additional purple color in Magic? If not, why they ever made that back in the first place then?
Here you go.
Fully-powered 600-Card "Dream Cube" https://cubecobra.com/cube/list/dreamcube
450-Card "Artificer's Cube" https://cubecobra.com/cube/list/artificer
Cubing in Indianapolis...send me a PM!!