That pic of the three I just added the other two pennies in the pic to emphasize the difference in color, the arrow points to the penny in question! Also It's off Center strike a smidgeon o the obverse, and the rev shows signs of doubling especially in the words United States of.. one And a few letters in cent?
As far as the color goes, copper is a very reactive metal and it can take on toning/shading in a variety of ways, depending on what environmental agents it's exposed to. The 1991 example also shows a minor misaligned obverse die. The reverse may be showing evidence of die deterioration--not to mention zinc rot, which may be contributing to misshapen lettering/devices.
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