What is the color of the circular background of the official Air Force seal?

The official Air Force seal features a circular background that is colored ultramarine blue. This specific shade is distinguished by its deep and vibrant hue, which is historically significant and often associated with the U.S. military's identity. Ultramarine blue not only provides a striking contrast for the elements of the seal but also symbolizes loyalty, trust, and the vastness of the sky, reflecting the Air Force's mission.

Other shades of blue, such as sky blue, dark blue, and royal blue, do not match the precise color designation used in the seal, which has been carefully selected to represent the values and ethos of the Air Force. Thus, recognizing ultramarine blue as the correct answer illustrates an understanding of the symbolism and history behind the Air Force's visual identity.