Chocolate Wheat Pennies?

Well, it basically depends on how it was circulated, the condition, amount of wear, etc.

The coin that is in the center of your cents has likely seen less circulation, and has some original luster left.

The coin on the right(middle) appears to have been in something, and is darker than average.

The two on the left are more or less, avg. circulated, and have a normal brown color.

The topmost coin may have been cleaned lightly at one time, and does not have normal color.

The coin on the lower right has seen wear and has the right color.

There are really 2 reasons, the first is how much environmental wear the coin sees. for example, was it ever in dirt? or water, or even something like soda pop.

Secondly the cents are also a mixture of metals, and sometimes this mixture is not exactly right, so the coin may have dark spots or stripes, this is call wood grain or woodys.