The red NM ('Hatch') chilis are the result of additional ripening before harvest, whereas the greens (from same plant) are harvested earlier.
That makes sense. Here is another way to get red from green..
I lived in Santa Fe NM for 18 years. I loved the smell of chile roasting. When I asked the people doing it, why a few of the chiles were red, in a bag of greens, they told me the green chiles turn red on their own, after being collected green, if they are not processed promptly. Either way, I agree the red is a favorite, though I consider green sweeter and red more musky. I started out favoring green, and over time came to prefer red. Either one can be equally mild, or hot and spicy, that depends on the strain, not the color.
So my interpretation of that info, is that if you harvest green chiles when they are green, and store them, such as in a dried ristra, they will turn red.
Therefore, I consider NM Green Chile as being made from fresh Peppers, and red chile as generally coming from dried ones, hence the difference in taste..
Red, Green, or Christmas is the way restaurants offer it, Christmas, for the uninitiated, is 50/50 red/green