Very green hay often means its been harvested early in the sesson and brought in very soon after cutting, then been fan-dried in barns. This has both advantages and disadvantages. Hay that’s been left out to dry in the sun for 5 days has higher levels of vitamin D, and some say sun-drying  makes the hay a less ideal growth medium for aspergillus fungus. A human cant see or smell aspergillus, but hay producers tend to send samples out for lab testing to make sure their hay is good.

You could email Oxbow and ask. At least they used to be very good about getting back to customers that asked them things about their products.

People tend to prefer green hay over more yellow hay. Some hays are even dyed green to appeal to customers, but I don’t think Oxbow dyes their hay.