So my friend has the new Arcane Wizardry deck from Commander 2017 which is Blue, Black and Red. I have the Feline Ferocity deck which is White and Green so neither of the decks share a color identity. My friend cast Spelltwine and took one of my Green spells from my grave yard, Harmonize, and wanted to draw three cards. But since Spelltwine specifically says that you can "cast" the target cards, I thought that he would not be able to cast it as it doesn't share a color with his commander. Can someone please help?