Which of the following colors is NOT part of the United States flag?

The United States flag consists of three colors: red, white, and blue. Each of these colors holds significance in American symbolism, where red often represents valor and bravery, white signifies purity and innocence, and blue denotes vigilance, perseverance, and justice. The visual design of the flag includes 13 stripes alternating in red and white and a blue field with 50 stars representing the states.

The choice of green is the correct answer as it is not included in the design or symbolism of the U.S. flag. This distinction is important for understanding the flag's design and the meanings associated with each of the colors that are indeed present.