Earlier quoted context omitted.
There is an "extension" to .obj where vertex colors are added to the v line, like v x y z r g b But this stores a color per-vertex. Vertex colors in Blender, like UVs, are per-corner-of-poly. The ideal way to store vertex color information in .obj would really be vc r g b p v1/vt1/vn1/vc1 ... As it is, exporting per-vertex colors would require splitting a vertex into multiple vertices, one per distinct vertex color t…
That's what you do with normals anyway, right?
Normals are specified per-corner-of-poly in OBJ, just like UVs.