The sky is blue because of basic physics. The sun shines all the colors of visible light on the Earth, but the color we see as blue is made of the shortest, smallest wavelengths. For this reason, it scatters the most when hitting air molecules in our atmosphere, making the sky appear as blue due to the greatest scattering of blue’s short wavelengths.
Your blood is red because of iron and oxygen. Within your red blood cells is hemoglobin, a protein made of an iron-based compound called heme. Heme binds with the oxygen you breathe, and the oxygen-iron bond reflects light to appear red. Elsewhere in the animal world, blood can be yellow, green, blue, or purple, and in the “Star Trek” universe, Mr. Spock’s is green, because Vulcan blood is copper-based rather than iron-based.
Grass is green because of chlorophyll. Grass, plants, trees, algae, and even some bacteria have the impressive ability to make their own food out of sunlight, water, and carbon dioxide. The pigment that does this work is chlorophyll, which reflects green light and so appears green to us.
Water is not blue because it reflects the sky, but appears blue for a similar reason that that sky does. The water absorbs long-wavelength yellow, orange and red colors and reflects short-wavelength colors, mostly blue. Since blue is the color reflected back to our eyes, we see the water as blue.
This same reason our sky appears blue also explains the sun’s appearance from Earth. Our sun is white when seen from space, but our atmosphere scatters the sun’s shorter-wavelength blue / indigo / violet-range colors such that longer-wavelength red / orange / and yellow colors within sunlight reach us more easily, and the sun usually appears as one of these colors.
Incoming light refracts within water droplets while they’re in the air to separate the colors which compose white light, all of which move at slightly different speeds. With the droplets acting like a prism and a color separater, we can then see those individual colors as a rainbow. In addition to familiar rainbows, there are also “moonbows” and “fogbows.”
Hair follicles produce less color as they age, and the result is that your hair eventually appears white with no color to change it…unless you add some from a bottle. However, genetics and disease also play a role in hair color.