Where in the book of Matthew does it say Jesus skin is bronze?

There are no reliable sources to confirm that Jesus skin is bronze, nor does Matthew discuss what color Jesus is. However, realistically, Jesus was not white (Caucasian). As a middle-eastern male, he would have had skin that was common to the Semites-- it is closer to the color of people in India.

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There is however a reference in the book of Revelations that says His feet were like brass...

"His feet were like fine brass, as if refined in a furnace, and His voice as the sound of many waters" (Revelations 1:15, NKJV).