Color Systems Help Sheet
We first started with the Elementary color field shown at right. This is
not a true subtractive color system because when the primary colors
(red, blue, and yellow) are added together you DO NOT get black as
a result. You get a muddy brown color (these colors do not
SUBTRACT or ABSORB all light that hits them).
We studied this color field to help define a subtractive system because
you had already studied it in elementary school, so it was familiar.
You also learned about tertiary colors—the sum of a primary color
and a secondary color near to the primary on the color wheel…
BTW, the names are a bit off on the color wheel at right, the primary is
always named first and then the secondary (e.g. blue-violet and redorange would be the proper names).
Complementary colors in a subtractive system add up to black (or muddy brown in the elementary color field). Complements are
always across from each other on the color wheel, but you can also see that they are a secondary color and a primary color
that is not found in the secondary. This means that two complements have all the primaries added—no wonder they will add
up to black in a real system! For example: red and green are complementary colors; green is blue and yellow; so, red + green
is equivalent to red + yellow + blue.
Next, we looked at the true subtractive color system. The CMYK system. Mix cyan, magenta, and yellow inks
or pigments and you do get a true black ink or pigment. All the light that hits a mixture of these colors is
SUBTRACTED (ABSORBED) by the inks. This system is used by the color printer you may have at home. It is
also used in printing books, magazines, junk mail, product labels, packaging, and virtually anything else
that’s a colorful printed page or object.
Even though all three inks added together give a black ink, printers and printing presses usually use
black ink on its own. The reason for this is three-fold:
1) It would cost three times as much to print three colors of ink (and with most of the printed page
being black text most of the time this would get really expensive).
2) Three times the ink on the page would cause the paper to get a little soggy, and the images would
blur as the inks bleed into the fibers of the paper (unless you always use glossy paper).
3) Inks from a printing press are really layered on top of each other, so they really don’t mix. The inks need to be truly mixed in
order to absorb all light that hits them. This isn’t as true for your home printer, as you usually use cheaper paper that does
allow the inks to mix a little.
The lab also talked about why a red object looks red—it reflects the red light that hits it and absorbs all other colors of light that
hit it. This means if you shine green light on a red apple the apple will appear to be black. The green light is absorbed so we
cannot see it, and there is no red light for the apple to reflect toward our eyes. Since no light heads toward our eyes in this
situation, we see it as black—the lack of light.
Next comes the additive system (or RGB system) for light. This system is mainly used by anything that
has a light emitting screen (TVs, computers, cell phones), but it can also apply to things like stage
lighting for plays. This system has red, green, and blue as its primary colors. Since this system adds
light together from light sources, the primaries do no add together to give black (the absence of
light), but they result in white (all colors of light) when added. Complements will also add to give
white light in this system.
Notice something similar about the two systems? One system’s primary colors are the other system’s
secondary colors (and vice-versa). The RGB system has CMY for its secondaries—the CMYK
system has RGB for its secondaries. This is not a coincidence, as the two systems are just opposites—one adds light, the
other subtracts it!
Next we looked at filters—the key thing to remember here is that just like a water filter lets water through and nothing else, a
perfect green filter should let green light through while absorbing any other colors of light that hit it. The lab should have
shown you that no filter is truly perfect, and that every filter lets a little light through of some other colors.
At right is a spectrum graph for a very good red filter—about the best red filter
you can buy. The higher the black line, the more of that color light gets
through. Notice how 85% of the red light that hits this filter gets through? But
also notice how almost all orange light hitting the filter gets through, as well
as a little yellow, and about 15% of the violet light that hits it…definitely not a
perfect filter.
We’ll use perfect filters in most problems we do in class, but I wanted you to be
aware of the real world here. It’ll be important when we talk about the eye and
how we see color in a later unit.
For the reflections part of the lab I wanted you to see that we oversimplify things in Physics when we don’t need to be exact.
Earlier in the lab we talked about how a red object looks red because it reflects red light. That’s true, but it’s not the whole
story…look at these objects:
We would call each of them red, but they’re all different shades of red. Even the two cherries are totally different colors. What
does this mean? It means that red objects reflect some other colors too. The cherry at the far right might reflect a little orange
and a lot of yellow light that hits it, while the darker cherry doesn’t reflect as much red, no orange or yellow, and maybe some
blue and violet light, though not much. Just like there’s no perfect red filter, you’d have a hard time finding a perfectly red
object!
The last section on the TV Screen (though we used a computer screen), shows how the additive
color system is used for light emitting screens of all sorts. At right is a magnified image of the
pixels of RGB used in several types of displays. Combinations of only these three colors form
every image you see on a TV or computer screen. These dots of light are too small to see
without a magnifying glass nowadays—Apple even calls its displays “retina” displays because
the pixels are too small to see easily even with magnification. You can use what you know
about the additive RGB system to predict what pixels will be lit up for different colors…for
example, to get yellow on your TV screen, the red and green pixels must be lit in the spot that
should be yellow.