This olive colour were made thanks to weld (Reseda luteola).
Weld is a very old dyeing plant – it was identified on textile fragments from the Iron Age and possibly it was used even in the Neolithic period. It was one of the favourites dyestuff in Anglo-Saxon England, known also by the Vikings, still widely used in medieval Europe and even later, up to the introducing modern chemical dyes.
Weld on its own gives nice bright yellow, but it can be also easily modified by adding metal salts. In case of this green, we just added scrap iron to the pot. This method of modifying the colour was also practised by Medieval Age dyers. They could also use water from the quenching bucket (received from the local blacksmith!).