Color blindness
Color blindness is very sensitive to changes in material. A red-green color blind person may be unable to see the difference between colors on a map printed on paper. The same map on a computer screen or television may appear normally. In addition, some color blind people find it easier to tell the difference between colors on artificial materials, such as plastic or in acrylic paints, than on natural materials, such as paper or wood. Thirdly, for some color blind people, color can only be distinguished if there is enough color: thin lines might appear black, but a thicker line of the same color can be seen in the correct color.