Hi, I have a pretty average sized gold collection, and i have many pieces that have very different color. Basically, the two types i come across are this almost orange yellow glow color and then the golden brown color... sorry that these aren't the best pics, I am getting angry at my scanner right now, trust me. thank you for your help!
Based on the small photos: The coin on the right (1908) has nice original luster. I can even see a cartwheel. This is very desirable. The coin on the left (1910-S) has subdued luster. This coin may have been handled. Of course, the photos may be deceptive. Lighting, color adjustments, etc. can make a coin look better or worse than it really is.
I honestly can't tell from the photos either. The one on the left looks more natural IMO, but both seem dull.
The dullness is very likely from the scanner. they are both very bright pieces; when i get a camera you'll get to see that. whenever i DO get a camera. haha
A cartwheel is luster that moves around a coin as it is rotated. It is called a cartwheel because it looks like a cartwheel (a wheel on a cart) that is spinning. Next time you are in a coin shop or at a coin show look at several uncirculated Morgan Dollars or other large coins with good luster. You should begin to notice on some of the coins that the luster moves around as you tilt the coin at various angles. This site - Coin Centric has some decent info on luster with a few photos showing luster (but they don't have a photo showing a cartwheel).
I would imagine that the lower purity of gold in the American Eagle coins show toning of the alloyed metals. When I get some gold, I'm going for the .999 pure stuff.
I think you're right. Gold coins for circulation needed 10% copper; pure gold is too soft. The copper tones in various ways, but it's subtle.
Circulating US gold coins were 90% gold and 10% copper. American Eagle Gold Bullion is 91.67% gold, 5.33% copper and 3% silver. So the AGEs have less copper than the circulating gold.
It is quite natural for gold coins from different years to have different colors. Rarely are they the same. But most from a given year will have the same basic color characteristics as all others of the same denomination from that year.