For example the dust balls rolling on the floors are indeed usually gray in color. Dust consists of, for example, hair, animal hair, textile fibers, insect parts, human skin cells and small dirt particles.
Many parts of the dust are naturally gray or dark, but there can also be very colorful bits, especially from clothes, towels, blankets and sheets. However, grayness or darkness is the main part.
When you look at the color spectrum, all the colors come together in the eye and the whole looks gray.
In a similar way, for example, sunlight looks white, even though it actually consists of several colors. Sunlight can be broken up into its colors with a prism device, and the different colored parts of the dust are best revealed with a microscope.
Tom Kuusela
University researcher in physics
University of Turku