Colors must have sufficient contrast between text color and its background (technically called luminosity contrast ratio). This includes text on images, icons, and buttons. Colors used to convey information on diagrams, maps, and other types of images must also be distinguishable.
This is especially helpful for individuals with color blindness. Color-blindness does not literally mean that a person can't see any color at all (except in very rare cases). Color-blindness refers to the inability to distinguish between certain kinds of colors, especially colors that are of equal brightness or luminosity, even if the colors themselves appear quite different to people without color-blindness.