Geiz isn't orange though. It'd be odd for only one Vistamp to reference a secondary rider

That’s the only idea I have. And the colors match the visors of Zi-O and Geiz for some reason.

I personally feel it's Grand Zi-O

I can see the Zi-O statue on his head becoming the bump/fin on a Brachiosaurus head. Though I wouldn't be surprised if its based on the TimeMazin and used for Revices mech form

It is confirmed to be the mech form, but that's part of why I'm thinking it's Grand

That makes sense.

I’m pretty sure that Green/Black is commonly used for W’s Legend Rider collectibles. The Gashat and Ridewatch come to my mind. Strangely, they used purple instead of black for Eagle.

I think there's enough purple in the Joker side of W's suit to justify it.

The lights from the Joker Memory are also purple, and a lot of other things like effects are associated with it as well. Green/Purple just also stands out a lot more and is more unique to W than Green/Black, which could apply to a lot of other Riders (especially Showa Riders)

The Ridewatch was purple and green