In my opinion it's WB -> UR -> BG -> RW -> GU since you can see every colour as a number modulo 5, so if W=0, U=1, B=2, R=3 and G=4 the enemy colour pairs are (0,2), (1,3), (2,4), (3,0) and (4,1) and from each one you obtain the next by adding +1 (mod 5)