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.