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Heat and tumbling in the dryer will contribute to fading. Buy a rack and hang your clothes to dry inside. Lay wool items out flat on a towel to dry them. Your clothes will usually smell fresh and retain more color than they would with constant drying cycles. Overdrying will beat up the clothes and fade them prematurely. Direct sunlight will start to bleach the fabric almost immediately. The closer you are to the equator, the stronger this effect will be.
Hang dry all your colored garments, if you have the space to do so. Line dry in out of direct sunlight during the warmer months. Set the dryer to the lowest amount of time possible. Change into different clothing if you are in bright sunlight all day.