What's a good way to have every tri on a sphere's surface know which hexagon/pentagon cell it's in?

“In your first paragraph alone there are 5 places where you refer to a thing that you haven’t actually written or named”

Very first line of my post: vertex paint.

How is it I haven’t named the thing when its literally the first line? :shrug:

your question should be “is this avaliable to access over BP”.
in which case, it wasn’t by default a while back / you have to write your own CPP function to sample FColorVertexBuffer.

However, put that aside. The concept is the exact same.
if you color the original sphere manually in a DCC, then subdivide it, you retain the original colors on the vertex paint in each square.

you can leverage this to parse all the faces and output an ID and a Color in text file.

you can then use thebfile inside unreal to manage whatever / however.

Mind you, it’s actually not clear at all what you wish to manage and how, which is mostly the issue.
you can get the data a lot easier in a 3d program then you can in engine simply because you’d have to script a cpp function to read vertex color values when really, all you need in the end is a flat text file to fill an array with / do other data stuff (we readers would assume)