I suspect that this 1999 cent has either been dipped in something or treated in some other fashion. I say this, because even though the obverse and reverse are a nice brass color, you can still see some copper what appears to be copper coloring on the very edge of the rim. The outer rim is also a nice brass color. The photos don't show the true color of the coin. Thanks Dave
they are plated steel, the new cents, this one was probally in contact with some sort of corrosive agent
I think this was plated with something. When I was in high school we did a plating experiment with a Lincoln cent and zinc. We stripped the zinc off a galvanized nail and then plated it onto the cent. I seem to remember I was surprised when they ended up a tinted yellow rather than a silvery color. I assumed it was due to impurities.