I don't think they are machine painted : I belive that can be done for things like markings on model aircraft , but I'm not sure it's possible on an irregular surface like a model head.
I believe they are hand painted by workers in the Chinese factories where 1/6th figures are made: there is often some variation if you compare several heads, though not much.
I'm sorry some people don't like them : they have immense possibilities, because once you are fed up with one view, YOU CAN CHANGE IT !