Why does red and blue pigment make purple?

I’ll start out with a disclaimer.  I am not an artist.  I understand that mixing red and blue pigment (or paint) does not produce a true purple.  I’d like to talk about the science behind what is happening in our eyes when we perceive color.  I’ll leave discussion of the color wheel, the true primary colors and what color is produced when red and blue paint are mixed to the artists!

It’s important to talk about the difference in how colors are produced by pigments and how color is produced by light.  Let’s start out with light.  

The visible spectrum of light goes from a wavelength of 400 nanometers (nm) to 700 nm.  I remember learning the acronym ROYGBiV (Red Orange Yellow Green Blue Violet) in school to remember the sequence of colors in the visible spectrum.  Red light has the longest wavelengths (from about 575-700nm) and the lowest amount of energy.  Violet light has shorter wavelengths (about 400-435 nm) and much greater energy.  

When we see white light, we are actually seeing a number of colors of light which together produce white light.  Natural light (sunlight) contains all the colors of visible light.  This is why you see the entire spectrum when you see a rainbow or when you use a prism to separate the colors from sunlight.  I always remember the album cover from Pink Floyd’s The Darkside of the Moon.  Fluorescent lights produce only certain colors of light, which produce a perception of white light when combined, but we can sense that the color of the white light from fluorescent lights is just not the same as natural light.  What’s the take-home message?  When you see all colors of light at the same time, your brain perceives this as white light.

Our ability to see colors is the result of receptors in our eyes called cones which are sensitive to the colors red, blue and green light.  

The story about pigments (mixing paints) gets a bit confusing.  We see that pigments have colors due to the colors of light they absorb or reflect.  A red pigment reflects red light and absorbs other colors.  The red light that makes it to our eyes is picked up by the red cones and they send a message to our brain that we are seeing something red.  

Something which is black is the result of pigments which are absorbing all colors of light and reflecting none.   The absence of light of any color is perceived by our brains as black.

A white pigment or paint absorbs no colors of light and reflects all colors.  What reaches our eyes is white light.

So why does red pigment when mixed with blue pigment produce something which looks purple?  The red pigment reflects red light, and the blue pigment reflects blue light.  These colors of light will reach our eyes and activate the blue and red sensitive cones.  This dual stimulation is perceived by our brains to look something like a purple color even though no purple color light is being reflected by the mix of red and blue pigments!

This post was inspired by a question posed recently by one of my chemistry students.