If you ask Gamblin, the maker of Artist's Oil Colors and mediums, they'll say "It depends on the year."
To understand their answer you need to understand the process of making paints. Now hold on! Don't get your Bunsen burners all out of shape, this is not going to get real technical or scientific. You see, Robert Gamblin believes "Pigment dust should not go into the earth, water or landfill, but into paint."
Every Spring they collect all of the pigment dust that their Torit Air Filtration system gathered that year and they put it into a very special limited supply vintage of oil paint. As you would imagine the mix of all these pigments would favor a grayish color, hence the name Torrit Grey. Torrit Grey is only available (free) until the supplies run out and slightly differs in hue every year. The folks over at Gamblin have started stamping each years' batch with a date to give you aficianados a reference for which you can catalog your paints. Upon the release of the new supply of Torrit Grey comes the annual Torrit Grey Painting Competition. Judged by Robert Gamblin, the entrants submit a painting using only that year's Torrit Grey and any black and white paint. Enter this year's competition here.
BTW Gamblin Artist colors and mediums are made in Portland, OR USA
Personally I think this is a very responsible and creative thing to do with what would otherwise be waste. What do you think?