Everything You Need to Know about Color and Light.  By Don Finkeldei:  A short, concise, and to the point explanation for a lot of things you didn't know.

What is color?  The human eye (and brain) recognizes a certain range of radiation called the visible light spectrum.  That’s a narrow band of electromagnetic oscillating radiation from about 390 nanometers in length to about 750 nanometers in length and travels at the speed of light.  Don’t worry about what a nanometer is ( It’s a very small distance), or the speed of light (it’s very fast).  The point is that these oscillations register as color to us from violet through blue, green, yellow, orange and red (and back to violet)  going from what we call cool to warm color temperatures.  Blue is cooler than red and yellow.  Red and yellow are warmer than blue.  Green is cooler than yellow, orange is cooler than red and so forth.

There are only three primary reflective colors.  They are pure red, yellow and blue.  Any other color across the spectrum can be made from a mixture of only the three primaries.  There are many secondary colors which are different combinations of any two primary colors.   Different amounts of red and yellow (the oranges), yellow and blue (the greens) and blue and red (the violets).  In addition there’s a wide range of tertiary colors.  A tertiary color is any combination of color composed of all three primaries (red, yellow, blue).  A tertiary color is also called a gray color.  Gray doesn’t have to be a neutral gray without color but can be a chromatic color such as a red gray, or orange gray, or yellow gray, etc.  Any gray by definition contains all three primaries and can vary in different proportions.  It’s as simple as that.

There are two primary types of light: Additive and subtractive.  The light coming from a source of light like the sun, light bulb or television screen is additive.   It’s always much more intense than the light produced by a painting (which is subtractive).  Why?

Additive light from a source like the sun is additive because it takes the full spectrum from red through orange to yellow, blue and violet to make an intense white light.  A prism will separate the sun's colors into a nice rainbow spectrum as you've probably experienced in a science class experiment in grade school.  A television screen uses colored phosphors of only three colors (red, green, blue) to approximate the full spectrum.  Stimulating all three phosphors to radiate equally produces the white on a tv screen.    Subtractive color means that a surface (like a painting surface) absorbs all other wavelengths of light and only reflects the color you painted the passage with.  

A painting only reflects light.  It’s not a source of light like a tv screen or the sun.  Paint on a canvas absorbs all light hitting it and only reflects and re-radiates the light of the color of the paint.  Red paint absorbs all wavelengths.. all blue, yellow and everything in between) and only reflects red which is reflected back to your eye.  Orange reflects back to you only combinations of red and yellow….. and so forth.  Hence, the intensity is much lower because there’s much less energy due to the fact that most of the light is being absorbed and not re-radiated back to you.  

Another thing to consider when painting is the type of light hitting it.  If you have a florescent (bluer, cooler) light on the canvas (or other warm or cool lights) the effect will be different than what you'd get painting it under different lighting conditions.  If you paint a canvas under a strong blue florescent you will not add enough blue in your painting to make it read right under an incandescent (warmer) lighting situation.  If you paint under a strong yellow incandescent light you won't add enough yellow to make it read right under a cool florescent light.  Normally, ideal lighting in a room or gallery is a combination of warm and cool lights that approximate a shaded canvas and palette outdoors.   That’s why it’s important to keep your palette and painting shielded from the sun while painting plein air.  A shaded palette and painting is closer to the conditions of indoor lighting.

The sun, as you've probably noticed, is not white.  It's yellowish.  That's because the atmosphere scatters a lot of the blue from the sun's spectrum.  That's also the reason the sky looks blue rather than black.  The atmosphere contains molecule and atom sizes that are prone to scatter the blue.  The sun viewed from space above the atmosphere is white hot.  The sky is black.

Mars has a very thin atmosphere of mostly carbon dioxide and dust particles.  It  has a butterscotch color during the day and it's not as bright as our blue sky.  At sunset on Mars, the sky is bluer near the sun and tends toward pink further from the sun.  The moon has very little atmosphere of any kind.  The sky is almost black and the sun is almost white.  This has nothing to do with painting landscapes on earth - but if perhaps  someday you find yourself setting up plein air on Mars or the Moon, keep this in mind.