Have you noticed that there is a difference in perceived brightness in primary colors? Take a look at this picture. It is supposed to look like six colored circles in bright light, with a shadow falling on parts of them. Do the bright colors appear similarly bright to you? Do the darker colors in the shadow appear similarly dark? Click the stage, and you'll find that the bright yellow and cyan, and the dark blue, are all primary colors. All other colors have reduced saturation or brightness. Click again, and you'll see the primary colors laid on top for comparison.
