'vergaut'? Blue and yellow make green!

The term 'vergaut' was introduced to me by Elena Wyth when she asked a question about mixing indigo and orpiment on Facebook. I hadn't come across it before, so I went to the books!

There seem to be many medieval instructions for mixing orpiment and indigo but few call it vergaut, as far as I can see. Read more below to see what a brief search helped me find.