The case
You saw the droid on lunchboxes, bedsheets, action figures and the movie itself, and you remember gold. Look again: below the right knee, Threepio is silver. It was there in the 1977 original and stayed through Return of the Jedi.
The toys and much of the merchandise painted him all gold, and lighting in many shots washes the difference out. So the mass-market version of the character was gold, and the film version quietly wasn't.
Why our brains do this
Merchandise simplified the design, and the simplified version was the one you handled every day. The screen detail lost to the toy.
Sources: Wikipedia — C-3PO · Wikipedia — False memory: the Mandela effect
