Green hay is generally considered a better quality than golden hay (with the exception of oat hay). This is not an absolute truth though. There are unscrupulous hay merchants that even dye hay green.
A friend of mine raises race horses. He grows his own hay for them and lets it dry for 5 days in the field. It gets a little more yellow, but the risk of mold is lower than for hay thats collected sooner and dried inside the barn with big fans. Direct sunshine makes hay more yellow, but it also causes vitamin D to develop in the long strands of mown hay. My friend sends his hay in for analysis, to make sure that his horses only get top quality feed.
Hay is something of a science in itself. One batch is not like the next batch. We bun owners often have to try a variety of hays until we find one that our buns approve of. And then next year’s hay from the same supplier can be so different that the buns just “Duh, like I’m eating this 🙄”
It’s not possible for the human nose to identify mycotoxins (toxins produced by certain types of mold). Hay can look and smell perfectly fine to us, but rabbits have a vastly superior sense of smell.