In Star Wars, C-3PO is a gold robot but his right leg is silver. Why is that?
Interesting question. I encourage you to read up on it on Wookieepedia.
In short, at the end of episode 3, C-3PO was dipped in gold and had his memory erased. He then served as a translation droid along side the Organa family in the senate.
Some time between then and the events in episode 4, a bomb was placed in his leg. Events unfolded that allowed the safe removal of the leg, but as he was dipped in gold specially, no gold legs were around. And time (or hard times in the Organa family) never permitted him to dip the leg.
That's the canon. But the real reason is simple. They wanted him to look like he had some history. Like a scar on the face.
It's not explicitly stated why his leg is silver, but in the Phantom Menace there's a nice little "blink and you'll miss it" moment where you see Jar Jar (in Watto's Shop) pratfalling in front of a set of droid coverings that are missing a leg.
It's certainly possible that these were the same covers used for C-3PO, with the absent part replaced by one from another droid. This would explain why the gold paint put on by Padmé wears off at a different rate, leaving one leg silver.
C-3PO's leg was replaced with a dummy containing a bomb, as seen in the Star Wars Comic serial "Star Wars: Droids: Rebellion"
There's a second (albeit slightly contradictory) account of this incident in "C-3PO: Tales of the Golden Droid"
Greek's trail led to the notorious Smuggler's Moon of Nar Shaddaa. Unit Zed, a police droid, was authorized to operate autonomously, but the slum dwellers of Nar Shaddaa had no respect for law officers, robotic or otherwise. Unit Zed was blasted to scrap and See-Threepio lost his left leg below the knee. A mysterious droid named B-9D7 became Threepio's benefactor, replacing his missing limb with a new silver leg, but this foreign hardware was a curse, not a gift. A hidden programming module superimposed a confident, heroic personality over Threepio's fainthearted and fearful one and caused the droid to incite a robotic revolution against the droid manufacturer Boonda the Hutt.
The revolution came to a quick end when it was discovered that B-9D7, actually the human industrialist Movo Brattakin in the body of a droid, had orchestrated the events to eliminate his business rival Boonda. When a sleeper bomb was discovered inside Threepio's leg, the offending limb was replaced with a golden spare and the droid's original personality programming was restored.