i’m not aware of documentation. i know it works for blender imports tho. demo snapshot of the very application.
if you use attributes in blender (this is 3.6.5) you could write a python script and convert 4 attribute channels to a combined vertex color that unreal guaranteed imports and handles correctly. not sure rn how 4.x does that. it doesn’t have the color seperated anymore. i reckon the scripting thing would be the way to seperately author and combine before export. the workflow.