This olive colour were made thanks to weld (Reseda luteola).
Weld is very old dyeing plant – it was identified on textile fragments from the Iron Age and there are theories that it may have been used even in 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 itself gives really nice bright yellow, but it can be easily modified with addition of some minerals. In case of this green we used scrap iron, like rusty nails. It was one of method of changing colour used by Medieval Age people. To receive similar effect, they could also go to blacksmith and ask him about some water used previously (for quenshing) to cool down freshly made iron items!