Ron Guth: In 1942, the Mint made Jefferson Nickels out of two different compositions. Those made early in the year contained pure Nickel. Those made later in the year consisted of an alloy where the Nickel was removed completely and replaced (in part) by silver. The reason for the replacement was because of the need for Nickel by the military in World War II.
The colors of the two alloys are similar, but different enough that the coins can be told apart by the naked eye. The silver alloy is a brighter white color when the coins are new. When worn, the silver alloy takes on a greenish color. However, just to make it easy to tell the coins apart, the Mints placed large mintmarks on the reverse of the coins, just above the dome of Monticello. This was the first appearance of a "P" mintmark on any US coin.
Both types of 1942 "Nickels" are very common and can be found in high grades with relative ease.