Natural dyes can change over time and depending on how they are used. Some tones become lighter, others darken and get a patina like wood or leather.
I work conscientiously, select dye plants with care, and go through many steps in the dye process to create lasting dyes.
And you can also help with this:
Wash them plant dyed goods by hand, with lukewarm water. If you need to use a washing machine, choose a short gentle cycle, preferably use a laundry net and a lower spin speed. Always use a gentle liquid laundry detergent.
Dry in the air, and not in the direct sun light.

When you are not using your textile, it is best to put it in a closet or somewhere similar. Permanent exposure to sunlight can fade natural dyes.