[Sunred]

So here's a thought. Zyuran and Vroom have yellow eyes while Gaon and Magine have green eyes. Zyuran and Vroom compose the left of Zenkaioh while Gaon and Magine compose the right of Zenkaioh respectively. Also, the primary two combinations if you think about it are dichotomies of each other's motif.
Zenkaioh Zyuragaon: ancient times VS modern times
Zenkaioh Vroomagine: science VS magic