Verdigris is basically very oxidised copper. If you scraped the green stuff off the Statue of Liberty and ground it up, that's verdigris. In the middle ages it was made by putting copper in a box with vinegar and burying it for a few weeks. Then they'd scrape off the layer that formed, bury it again, and repeat until the copper was gone. Whatever is painted with it turns a very striking light green. But that's not always what's wanted. It needs to be toned down a little unless you want your lovely manuscript illumination looking like a corroded statue.

The wort is a bit confusing, since wort is normally unfermented beer, which was sometimes used as an ingredient in ink, including in Harley MS 2253 - literally the next recipe involves rotten apple juice and is also referred to as 'wort'. But 'wort plant' probably means Hypericum perforatum, also known as 'St. John's Wort'. It's a common plant across Europe and has long been recognised as a highly versatile plant with a variety of uses. We now know that some of the chemicals in the oil are antiseptic, and it was commonly used by the Knights Hospitaller to treat wounds. This might actually be where the name comes from, since the Knights Hospitaller were officially known as The Order of St. John. Medieval medical texts refer to it by a variety of names, but that's the one that caught on. It also has a lot of other medical properties that meant it was used for almost every problem you could think of. It was basically a miracle plant, and it's all over medieval medical handbooks. Oil could be extracted from it, and it was cultivated in large quantities because it was an essential component in medieval medicine as well as a base for many inks.

The oil of St. John's Wort is a yellow-green colour. In combination with the striking light green of the verdigris it would, apparently, have produced the colour of grass. I'm not sure how you viewed Harley MS 2253 and if you saw the title of the heading, but the entry is called 'Vorte make gras-grene' or 'Wort made grass-green'. It would have been used in painting and for manuscript illuminations, especially for field scenes, but not for writing since black (or extremely dark shades of a colour) were almost universally favoured for text.

So the plant is probably St. John's Wort and the purpose was both to provide the liquid and to tone down the verdigris.