The different hues of the Golden Dollars now circulating are the result of the manganese brass contained in the outer layer of the new coins. Like any brass, its color will eventually become darker, giving your coins an antique finish.
The high concentration of manganese in the alloy compound contributes to the darker color of the coin, however, the zinc and nickel in the alloy inhibit this process somewhat, ensuring the new coin will not darken as much as pennies do.
As the coins are handled frequently, the darker “patina” may wear off the high points of the coin, leaving golden colored highlights that accent the darker background around the border, lettering and other less exposed areas. The brighter, brass highlights, in contrast with the darker background, accentuate the profile and add a dimension of depth to the depiction of Sacagawea and her child.
Regarding reports of spots on some dollar coins during the first few weeks of production, the Mint identified the cause as residue left on the coins during the manufacturing process and has eliminated it.