We know that natural light is called white light is composed many colors mixed together. Since the purple until the red light like the sun. The sense that we know as different colors because that color is reflected into the eye. In fact, any object that has no color in itself but an object has the ability to absorb certain color and reflect other colors. If we can find ways to separate the red and violet light from the Sun (white light) then we'll find that the remaining light is green. Similarly, an object was seen as yellow must have properties to absorb blue light. Older white fabric tend to be yellow color because it absorbs blue and violet light. The light that entering the eye will be yellow due to lack of blue and purple. Indigo, which we put into fabric to be blue color. It makes old white cloth to look whiter because this material absorbed yellow light and remaining part of reflects out of fabric to some extent mixed with the blue light that is reflected out of indigo. Thus we see is white.