From what I understand, it's normal for clownfish to change color as they age, and also in response to your lighting. Hosting in certain corals as well can make them darken. I know Fluff has had this issue, as well as a few others on AA.
They can also get darker from stress. If you water quality pans out as being fine, I would assume it completely normal, like the others said, and not worry about it.
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