Are Once-lers green all over? Are they even green at all? We have no idea. All we ever get to see of any Once-ler are green, "gruvvulous glove[s]" (41). Why? We do have an idea on this one: because it's all the boy sees of him. This makes us think that the Once-ler's flashback is shown as the boy is imagining it. Since all he's really seen of the Once-ler is green gloves, that's all he imagines. And of course he assumes the Once-ler's entire family looks just like the Once-ler. (Is he a racist? Or a… Once-list?)
But we're talking about symbols here, and it's pretty clear that the gloves are a symbol of greed. (Or maybe he just does a lot of dishes. You never know.) In case you've been in a hole your entire life, here's a newsflash: green is the color of money. The illustrations show that these gloves, or at least the pair worn by the Once-ler, have a "strange, secret hole" called a "Snuvv" (39). This is where he drops the filthy lucre that the boy gives him in exchange for his story.
Oh, but green is also the color of nature and environmentalism. Hmmm, what do you make of that?