# Color Redux

In Monday’s post I started writing about light and color. I described how white light can be created by adding three primary colors (red, green, blue), and how mixing any two result in secondary colors (yellow, cyan, magenta).

I went on to describe how subtracting two of the secondaries gives you the primary color they have in common, and how subtracting all three filters out all color, giving you black. The secondary combinations are the negative of the primary ones (e.g. blue is “anti-yellow”).

Finally I touched on how color is the “pitch” (frequency) of light. X-rays, radio waves, microwaves and gamma rays are all forms of light.

Today I continue the topic by exploring some details and nuances.

The primary colors:
Red, Green, Blue
(figure 1)

Let me start by emphasizing the distinction between adding and subtracting colors.

When working with light, almost always we add colors of light to create new colors. There may be some exceptions, but they would need unusual interpretations of “working with light,” so I’ll ignore them.

The place most of us experience this is every computer monitor and color phone or game display we ever see. The old glass CRT tubes did it, and so do all the new LCD flat panels. Those giant sports display boards and the new animated billboards use tiny (but bright) lights (usually LEDs) to mix red, green and blue.

Now mixing any colors of light always results in new colors. But if you want to be able to make all the colors as efficiently as possible, you need red, green and blue. If you don’t want to be efficient, you can use a crayon box with 16 million colors; three or 16 million, the choice is yours.

One proof of the primaries is that no combination of colored light can give us red or green or blue, whereas all other colors can be made from combinations. In all the display examples above, the displays use tiny red, green and blue “lights” to generate “all the colors of the rainbow.”

The secondary colors:
Yellow, Cyan, Magenta
Subtractive demo
(figure 2)

On the other hand, most other full-color phenomena are subtractive. Different colored filters remove certain colors from light to produce other colors. All color photos and full-color printed material (magazines, newspapers, posters, billboards) use subtraction. Mixing paint is subtractive. Each paint color removes certain colors.

Movies (if shown using a film projector) are subtractive. Movies projected with a digital projector work like your monitor by adding red, green & blue light. The old style digital projectors actually had three separate lenses!

The color of the world around you is largely subtractive. Things are the color they are, because they’ve subtracted (absorbed) the colors they are not and reflected back the colors they are.

A nuance here regarding subtractive color is that it is only as good as the illumination source. Subtraction always starts with white light and then removes color to get the desired color. But if you don’t start with white light, you won’t end up with the intended color.

Imagine you’re wearing a yellow shirt. It’s yellow because it reflects yellow and absorbs non-yellow (blue).  Yellow is green and red, so the shirt reflects red and green and absorbs blue. Under a red light, it will look red; under a green light, it will look green. And under a blue light, it will look black!

This all assumes perfect lights and perfect filtering. The blue light, in particular, is likely to contain other colors. True blue is a deep, dark blue. The light sky blue most people think of as blue is a pastel.

Photographers are very careful to view their work in light that is specifically white. Which brings us to an interesting technical point about “white” light.

The human brain/mind system is very good at screening out all kinds of “noise.” In this case the noise is slight variation in the color of light. We generally perceive sun light, sky light, fluorescent light, and incandescent light as white. We are able to distinguish colors just fine using those light sources.

In reality, they’re all different. Sky light (from a blue sky) is bluer than direct sunlight. In fact, children make yellow suns more in contrast to the blue sky than because the sun is actually all that yellow. We do live under a yellow star, but it’s not a banana star! (And we more “zip around it once per year” than “live under” it.)

Indoor lighting (the time-honored and now endangered incandescent) is actually much more on the yellow side than sunlight. Fluorescent lights are a sick torture device. They’re spiky light-wise, heavy peaks in the green and blue range. And they turn on and off 120 times a second. And they’re filled with poison.

The new LED lights I don’t know too much about, yet, so no comment.

The blue scary under the friendly white coating.

No, screw the no comment thing: I love my inkies and hate all that high-tech shit. White-hot tungsten in a fragile evacuated glass bulb; yeah, that’s the way to go, baby!  (And yes, I’m one of those who’s been stocking up.)

You may have heard that artists like a studio with northern light. There’s a very good reason for this. Two, really. The first is that northern sky light is more consistent throughout the day. East or west exposure gives extreme changes across the day, and southern exposure light also varies as the sun moves.

The second is that the northern light more closely matches the “standard white” that is universal among photography.

CIE chromaticity diagram.
The black line through the middle shows the “black body” radiation curve. The various “white” color temperatures are found along this line.

The yellow/blue variation of white light is called its color temperature. That incandescent lighting is about 2900°, sunlight is in the 5000-6000° range, and skylight starts around 6500° and goes upwards depending on where you look in the sky. Clear blue sky is in the 15,000°+ range.

The standard white in photography is that 6500° skylight.  (And incidentally, the temperature scale here is Kevin.)

This variation in color is exactly why there is “indoor” and “outdoor” film. It can’t do what your eyes and brain does. Outdoor film used indoors comes out looking very yellow, even orange. Indoor film used outdoors comes out looking blue. That yellow/blue difference is always there; we just don’t see it.

To wrap this up, you may have noticed that my theatre demo contained two screens. The left one (confusingly, the one on stage right) clearly contained the red, green, blue demo. It very closely matches the schematic version I’ve shown you here (figure 1).

The YCM Demo.
Note how dim the three lights are compared to the RBG demo. Also note the pastel shades where two overlap.
(click for big’n)

The other one… does not!

The reason is that the schematic of secondary colors (figure 2) shows subtractive mixing, which is what we need with secondaries. But the theatre light demo is an additive demo! That’s the difference. On that other screen, you’re seeing what happens when we try adding secondaries.

A couple of things stand out. The colors are much dimmer than in the RGB demo (we can call the secondary demo the YCM demo). The red, green and blue lights are full on, and the secondaries they form are full intensity. So is the white patch in the middle. This is because we’re starting with the parts and adding them to get the whole.

The RGB Demo.
See how bright the lights are compared to the YCM demo!
(click for big’n)

But in the YCM demo, each part is really two parts combined. That makes the places where just two overlap contain four parts, and the center patch contains six parts! Just three parts comprise full white, so four and six represent light that is “too bright.” (Meaning, if we set our cameras so the RGB demo white looks normal, the YCM colors are very washed out and “over exposed.”)

By reducing the light level, we can get normal brightness levels at the expense of making the individual lights look dim. But the real deal breaker here is that we can’t make red or green or blue this way; we can just make pastels.

Just as an example, consider cyan+yellow. Cyan is really blue+green, and yellow is really red+green. When we add cyan+yellow, we get red+green+green+blue. That gives us white with an extra green; in other words, a pastel of green.

And on that note, I’ll end for now. Next time I’ll pull out those colored filters you saw off to stage left during our tour yesterday. You’ve seen the additive demos; when I resume I’ll show you two subtractive demos. I’ll warn you now: one of them isn’t very thrilling.

Three ways of slicing the color cube of the 216 original “legal” web colors.

## About Wyrd Smythe

The canonical fool on the hill watching the sunset and the rotation of the planet and thinking what he thinks are big thoughts. View all posts by Wyrd Smythe

#### 2 Responses to “Color Redux”

• reocochran

Oh my goodness! You are such a genius and I understood most of this, ha ha! Have a great and colorful weekend! Try and stay warm, as I will too!

• Wyrd Smythe

[blush] Well, thank you! I’m glad you got something from it!!

Yeah, it’s 18 here this morning, and that’s apparently the high. Headed down to -3 for overnight low. But there does seem a bit of a bump coming Monday. A whopping 35 degree high! And then snow.