Which brand?
I'm sure it's the quality / material issue.
Quality issue: Cheaply made clothing always has the problem of color bleeding, especially Reebok kit provided for ufc fighter is famous for it.
Material issue: All of my mma shorts that has a white flex panel slightly turned into the main color of the shorts too (about 30%). Yet, White as the main color shorts aren't affected by it at all. It's just the surface of the material use for the flex panel and the majority of the shorts is different. Majority of the shorts are made with polyester, and the flex panel probably made with spandex or flexible polyester.
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Once they have turned, they don't get any worse though, stay the same reddish/grey/blueish color. So I just avoid getting mma shorts with white flex panel in the future. For thai shorts you can avoid certain brand or shorts that have white patches/badges/words that aren't made with Nylon/Satin material.