Instance material cost on mobile?

I’m having a hard time getting a distance colour fade working on mobile for my meshes (I’m doing it via a blendable post processing) and was thinking I’d just create material instance and tweak their material colours to fade out towards the sky colour in the distance. It might just work in my case although it’ll be annoying.

My question is, how much does it cost on a mobile to tweak a lot of material instances this way? How does Unreal handle storing this extra data? Will I get any extra draw calls per instance? Will Unreal still store only one texture in memory and handle colour changes somewhat cheaply?

Thanks.