这是因为在阳光下，真黑素被淡化，而红色则现显出来，虽然十分微弱。

The eumelanin has been bleached by the sun, and the red color is showing through, however slightly.

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黑素子类别中的褐黑素的多少会决定头发红色的程度，而另一类真黑素则会决定头发灰、黑、黄和棕色的程度。

The subtype pheomelanin is responsible for red hair, while the subtype eumelanin is responsible for grey, black, yellow, and brown hair.

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