Defictionalization: "green, egg and ham" was a real dish, but the "green" referred to a vegetable side dish rather than the color of the meal as a whole, hence the joke. While the real dish has fallen out of favor, actual dishes of green eggs and green ham, however, have cropped up as references to the book:
The Friendly Toast, a breakfast joint in Portsmouth, NH (with a second location in Cambridge, MA) is one of the more famous of the Greasy Spoon-type places that actually make their own version, though the ham isn't usually dyed green.
In 2012, as part of promotion for The Lorax movie, International House Of Pancakes offers Green Eggs and Ham, mixing spinach with scrambled eggs.
In addition, green eggs were annoyingly common in early Army rations. The sulfur in the eggs reacted with the preserving process, turning the eggs an unpleasant shade of green. The taste wasn't affected, but visually, more than a few soldiers found them disgusting.