Real gold does not tarnish. The metal is not susceptible to oxidization, meaning it does not turn brown, black, green or any other color over time. Dark spots or unnatural hues to a piece of gold indicate impurities and are common in jewelry that is of lower quality.
A flawless gold necklace, ring or other item retains its hue and shine but is subject to bending because gold is very malleable. While the metal is prone to deformation, gold is not a very reactive substance. That means it does not pick up additional electrons or other atomic structures, the process that causes rust and tarnish to form on other substances. When a substance is as inert as gold, there is no need to worry about discoloration in totally pure pieces because gold never interacts with other substances.
The more impurities in a piece of gold, the darker it becomes over time. Cleaning this tarnish is handled the same way as polishing silver, brass or other previous metals and indicates that the piece in question needs additional attention to maintain its overall appearance. Any piece of gold under 14 carats is likely to need this kind of care in order to keep it looking clean and new.