could be environmental issues, or a gold plated coin. Without a photo of both sides of it, it'll be hard to know what caused it.
It is probably Toning - the discoloration or light patina that forms on the surface of coins due to oxygen and chemicals in the air acting on the metal. This oxidation can result in a variety and level of toning and also depends on the properties of the metal. It can come in a variety of colors such as a gold tint.
Yeah, I have a Standing Liberty Quarter that is toned a really nice gold...it just happens sometimes.
I have several early 60's proof sets with varying degrees of gold tones, especially the nickels. The 60's were great for about every color of toning there is.