When it comes to natural dyes, there are a lot of factors that can affect how long the color will last. One of the biggest factors is whether or not you use a mordant. A mordant is a substance that helps the dye bind to the fabric, which in turn makes the color more light- and wash-fast. Many natural dyes require a mordant in order to stay vibrant for a longer period of time. However, onion skin dye does not need a mordant in order to be absorbed by the fibers. This makes it a great option for those who want to avoid using harsh chemicals or any extra steps. Even without a mordant, onion skin dye is still relatively light- and wash-fast and you can get a natural tie-dye effect. However, it is important to note that all natural dyes will eventually fade over time.