I was wondering why these card were printed in both Green and Orange backs.

According to the Sports Americana Price Guide to Non-Sports Cards, Series 2 cards had both the green and orange boxes on the backs. It doesn't say if one's rarer than the other, etc. Just a printing change? Reprinting due to demand?

Price wise there does not seem to be any diffrence between the green and the orange and the 2nd series fetches roughly the same price as the 1st series.

Source: The Gum Guide by Paul Hart.

It might not be reflected in the price but I have collected these cards for years and can assure you that the Orange backs are much harder to find that the green. Also, please be careful about buying these cards out of UK as a while back a seller was exposed for selling fakes (reprints) I've held one in my hand and it is extremely difficult to tell them from the originals. I got very suspicious when the card looked so "new" and took a known original with me to a reliable printer. He agreed that the stock used was not likely to have been around 50 years ago.