If you get an unusable 5* (meaning it would be a 6th for a color and the character is not at 13 covers yet) then you can open a ticket with customer support and ask for them to change it to a color of your choosing. After they confirm they will they will ask you to delete your cover from the vine and once they confirm delete they will send you the usable cover.
To open a ticket click the gears icon in the upper left-hand corner, click the help, then contact us. (at least on mobile)