mh, it seems the vertex colors need to be Color Structures and made of bytes rather than Linear Colors made of floats, however I don’t think you can actually create a Color Structure color as a variable? At least I’m on a Mac using 4.10, and I only have Linear Color as a variable option. Therefore I can’t make a vertex color array procedurally right now.
Found this: Struct Color and associated issue - Programming & Scripting - Epic Developer Community Forums with an answer from 2014 saying Color structure colors are absent.