Yes, Red + Green = Yellow. There is a very good reason why this sounds so counterintuitive and we’ll get to that soon.
“Ahem!” you say. “Lou, the information you are offering may be true for the physics of light but we are artists and use pigments. We work with subtractive (reflective) light not additive (direct) light. How does all this relate to us?” Excellent question. It may help to remember that all we see is light. Whether or not it’s reflective or direct we only process photons of light.
It will also help to remember the value scale (whether a color is dark, light or somewhere in between).* Yellow, as we normally consider it, is a very light value. Red and Green are both middle values.
French Chemist Michel Chevreul published ‘The Principles of Harmony and Contrast of Colors’ in 1839.* Chevreul discovered the concept of optical mixing and paved the way for the Impressionists who used ‘broken strokes’ of color side by side to create a third color to the viewer’s eye.*
Diagram 1 demonstrates the concept of optical mixing. Red and Green ‘strokes’ are placed side by side and the result is a third color. In the top row the red and green make a muddy color that most would not call yellow. But what happens when we lighten the value of the red and green in the bottom row?
Because Red and Green are middle values we get a middle value Yellow which we don’t normally think of as Yellow. Diagram 2 shows a color matrix with Green and Red at opposite corners from the top left to the lower right and Yellow and Black from the lower left to the top right.
That color in the middle is both dark yellow and red/green. This answers the second option in VLQ: What Hue is this?
So why is this all so counterintuitive? Well, it has to do with how we’re wired. When a photon of light hits our retina it fires a Rod or Cone.* But before the signal is sent to our visual cortex it passes through our Retinal Ganglion Cells. These cells are bipolar (literally a light switch) and they decide whether the light that hits that section of our retina is: Black or White, Blue or Yellow or Red or Green.** Black is the absence of light while white is the sum of all visible light. Blue is a primary wavelength of visible light while Yellow is the sum of the Reg + Green wavelengths. Red and Green are each primary wavelengths of visible light.
Diagram 3 show the retinal Ganglion primaries.
Still don’t buy all this? Well then a short video may be in order…
* We are jumping ahead here in many places. I’ll update this page with cross references as all these topics are documented.
** Yes, there’s more to all this from a biological point of view. This information is simplified so that it relates to the practical observations and actions of artists.