Since I don't generally stash, I organize by project. After I am done with a project I organize the "leftovers" by color and that becomes my stash. When I have enough leftovers or enough of a color that I am looking for in a project then the leftover gets moved to the project organization.
I do that with soup too. Whenever there are leftover veggies or meats that are not enough for one serving I put them in the freezer and when I get enough little bits of this and that, I make soup.