The colors we see in the sky come about from light scattering in the atmosphere. We observe colors when white light (from the sun) shines on a particle (gas molecule, dust particle, water droplets, etc.) and some wavelengths (i.e. colors) are scattered more than others. If all colors scatter to the same degree—then the object appears white (e.g. a white puffy cloud). The phenomenon is not limited to the atmosphere—it is also very common in biological systems. We see it in the colors of blue Morpho butterfly wings, colorful bird feathers, green lizard skin and countless other examples in nature. It’s called structural coloration and it is different than a pigment. A pigment is a substance or a molecule that absorbs light of a certain wavelength (and color) so that we observe the complimentary color. We have countless examples of this—including the chlorophyl molecule that absorbs red light which results in the abundant green color we see in plants. You can tell something is a pigment because if it gets on your hands or clothes the color transfers. The molecules will absorb light no matter where they are located.