Board Thread:General Discussion/@comment-36554724-20180814031505/@comment-28517148-20180815182703

''Well, remember what color soul Frisk's soul turned while fighting Undyne? Green.'' When battle ends by either win or loss, Frisk's Soul turns back to its original red color. The color assigned to certain soul mode doesn't stay on the Soul. In the other hand, if that color could stay on Soul, it means that all six Souls currently don't have their original colors, so they cannot by connected with the Ball game just like that. Your theory contradicts with the game.