Duffus often had friends over for tea, but most of them brought their kids, who had a tendency to eat any snack food found within a hundred meter radius. No matter where he hid his stash like starving piranhas they found and devoured his snacks. So Duffus being no duffis (well then again) devised a plan to save his snacks. He made up a game to hand out snacks to those who could guess the color of the marble he held behind his back. One at a time, a kid could enter the kitchen and guess, but they couldn't hear the others' answers. They could pick from red, green, blue, or yellow as the colors the marble might be. Duffus had the marble in one hand and would say, "When I show you the marble, what color will it be?" Duffus didn't switch marbles or pick a marble color other than the four stated. There was nothing on the table in front of him except his ice tea; Duffus loved his tea, though usually hot. When all the kids had a turn, every color had been picked, yet no one won. Duffus had not cheated and all the kids saw the marble after their guess. Thus Duffus saved his snacks - at least this time.
What did Duffus do?