I'm interested in Gamma World, and I see several different products out there. While I can see that e.g. "Famine in Far-go" is a supplement, I don't understand what's the difference between the "D&D Gamma World Box" and the "D&D Gamma World Role Playing Game". Is the first a reedition?
They are the effectively same thing. The D&D Gamma World Box link is a listing by an independant retailer.
The D&D Gamma World Role Playing Game is the amazon listing you want to buy. It's got all the core rules and setting in it.
The difference in colour is probably down to the retailer scanning in the book or box cover with bad colour correction. There's also no guarantee that Your Mom's Comics are selling the whole box, since that's missing from the description.