[Pineapple924]

I wanna say a brace, mainly because the show is about watches. Although that is assuming Toei takes the low hanging fruit.

Speaking of eye colors, what should the writing style for the tertiary rider be?

Zi-O is Katakana, Geiz is Hiragana, accounting to Wikipedia that just leaves Man'yōgana for the third rider, but there's still the chance they pick something else.