It's actually cabbage that is king green around most of the world for New Year's meals. Cabbage is a late crop and would be available this time of year. Collard greens are a late crop too, but they are mostly grown in the south. Traditionally, cabbage was picked and turned into sauerkraut. Sauerkraut, a fermented product, would just be ready to eat around New Year's day.
Cabbage and collard greens both represent "green" money in New Year's tradition, but, historically, cabbage was eaten for health benefits. Cabbage was eaten by everyone from Caesar to the Egyptians to aid in digestion and for nutrition, later for the prevention of scurvy. Aristotle, the philosopher, ate cabbage before drinking alcohol to keep the wine "from fuddling his prudent academic head." I wonder why we don't eat it on New Year's Eve? Eating collard greens isn't too far off from Caesar and Aristotle. The ancient cabbage those guys ate was probably closer to kale than our modern cabbage.
Collard greens (or any greens) sub for cabbage in the south because that's what we grow here in the late fall. The southern tradition: each bite of greens you eat is worth $1,000 in the upcoming year. I don't like greens much, so I'm surprised I'm not broke. Last year, I probably only ate about $5,000 worth!
Corn bread represents pocket money or spending money. It's another soul food we eat on New Year's. The tradition stems from the color of the bread. It's color represented "gold" or "coin" money. Plus, it goes well with collard greens, peas and pork.
We know what we're doing down here.