For dyeing, I use the outer skin of the onions, which can be stored or frozen if you are hoping to build towards a large dye vat. Yellow, red and shallots all produce stunning shades, ranging from yellow to rusty orange, tan and even a shade that is almost black, depending on your choice of mordant. Onions react well with other natural dye agents and yield unique shades if mixed with bark from trees such as alder or birch, and in reaction to a small amount of indigo.