I cast Refute to counter a spell and had six untapped islands available after the cast, in the discard selection I discarded Emrakul, The World Anew, it gave the option to Madness cast for six, but it would not allow me to pay using my six islands. As far as I know the 6 colorless mana is not restricted from being paid for by colored mana, I believe a bug has the system in Arena thinking that it specifically has to be “colorless” mana. If this could get resolved that would be fantastic.