During World War II, food was rationed, and everyone had ration books with "blue points' and "red points" and such (I can't remember now which colored "point" was for what kind of food). Fats were in short supply and butter was pricy in terms of points. But you could get margarine.
The dairy industry had managed to get a law passed (pre-war, I think) that makers of margarine could not color it yellow, because they could then pass it off as butter. So margarine (which was an unappetizing pasty white) usually came with a packet of food coloring that you had to mix in (pain in the neck). But during the war, some brands of margarine were sold in a plastic bag. At one point beneath the surface of the plastic bag full of white glop was a little thing about the size of an M&M that looked reddish-brown. You'd squeeze the M&M-ish thing until it popped (inside the plastic bag) and color would start to leak into the white glop. Then you'd knead the bag for awhile, distributing the color evenly until all the white glop had turned to a buttery yellow.
I was kinda young during World War II, and coloring the margarine was my job.