Which of these colors is not on a classic Rubik's Cube?

On the original classic Rubik's Cube, each of the six faces is covered by nine stickers, each of one of six solid colors: white, red, blue, orange, green, and yellow (purple is not a color on the classic cube). Some later versions of the cube have been updated to use colored plastic panels instead, which prevents peeling and fading. Since 1988, the arrangement of colors has been standardized with white opposite yellow, blue opposite green, and orange opposite red. On early cubes, the position of the colors varied from cube to cube.

Source: Wikipedia