How can paints be different colors? To make different colored paints, you need to add different materials for color pigments because different particles reflect different wavelengths of light.
Our eyes pick up reflected light using photoreceptors that are sensitive to different wavelengths. These then travel to our brain, which associates them with a color. For example, water absorbs long wavelengths and reflects shorter wavelengths of about 450 nm. Our brain associates 450 nm with the color blue, which is why the sea is blue. Plants absorb all of the visible light except for the band between 500 and 600 nm. Our brains associate that wavelength with the color green.
So, for paints to be different colors, they must have particles suspended in them that reflect the necessary wavelength of light. Our eyes can pick up wavelengths from about 450 nm (blue) to about 750 nm (red).
What is paint? Paint is a colored liquid that can be spread across a surface and dries to a solid form. Paint is made of four things: water, a binder, pigments, and additives. The water is used to dissolve the pigments and additives in. The water will evaporate away when the paint dries. The binder is what is left after the water has evaporated. It binds the pigments and the additives to the surface it has been painted on. The pigments give the paint its color and the additives make the paint smooth and dry faster.
Paints have been around for over 100,000 years. Pigments and paint grinding equipment that is over 350,000 years old was found in Zambia, so humans have been coloring things with paint for as long as we’ve been here. Prehistoric people living in South Africa used paints made from mineral and organic pigments to draw on the walls of caves. They used red iron oxide and charcoal and used bone marrow as a binder. The ancient Greeks and Egyptians realized that a better binder was needed and they started using beeswax, egg yolk, and milk.
So, how can paints be different colors? The pigment in the paint is what reflects that light waves and gives it its color. The concentration of the pigment makes the color darker or lighter and different pigments can be mixed to make other colors. Here are some examples of pigments in common paint colors.
The particles in blue paint reflect light with a wavelength of 460 nm, which we see as blue. It used to be made by powdering a rock called lapis lazuli, but that was very expensive. Only wealthy people could afford to paint things blue. That’s why the Virgin Mary is usually wearing blue robes in religious paintings. These days blue is made by heating china clay, soda ash, coal, silica, and sulfur in a furnace for up to 18 hours. It is then ground, washed, dried, and ground again.
The particles in yellow paint reflect light with a wavelength of about 580 nm, which we see as yellow. It used to be made with yellow ochre, which is an earth pigment containing hydrated iron oxides. During the Middle Ages, lead tin yellow was used, which is poisonous. These days it is made with nickel azo, Hansa Yellow, and quinacridone burnt orange.
The particles in red paint reflect light with a wavelength of about 750 nm, which we obviously see as red. It is usually made with ochre, which is basically clay colored with iron oxide. Many things in our world are painted red: barns, fences, and roofs. This is because red has always been the cheapest paint as iron is one of the most common minerals on Earth. Of all the pigments it is the easiest to come by. Iron oxide is also used to make orange, brown, and some yellows as well.
The particles in white paint reflect all of the wavelengths of light, absorbing none of them. Titanium dioxide is used to make white paint these days. When white paint was first used it was made by mixing calcite or chalk. The ancient Greeks made it using lead white, which is extremely poisonous.
The particles in black paint absorb all light and reflect none of it. (Technically, not completely true, because no manmade black material can absorb 100% of the light. The best we can do is about 99.8%.) This is done using powdered carbon.
You can then mix any of these colors to make any other color. There are thousands of different paint colors available to buy. It is questionable if our eyes are able to tell the difference between a paint that reflects 580 nm and one that reflects 581 nm, but the paint industry would like us to believe that we not only can, but it will make a huge difference to our interior decoration.
Sources
https://www.explainthatstuff.com/howpaintworks.html
https://www.thespruce.com/how-paint-is-made-4587730
https://www.andersonpaintingnc.com/blog/why-your-paint-color-looks-different-than-you-expect
https://sites.google.com/site/scienceofcolour/how-colors-mix
http://www.webexhibits.org/causesofcolor/7A.html
https://en.wikipedia.org/wiki/Paint