Have you noticed that there's always something new to learn? Furthermore, it's only when you are trying to explain something to someone else that you realize where there are holes in your own knowledge.
Just the other day, for example, I was chatting with my wife Gina about primary colors. Gina is incredibly good at interior decorating and using colors to their best effect. For my own part, based on my ever-evolving paper on Color Vision I flatter myself that I know the occasional nugget of trivia with regard to colors in general and primary colors in particular.
Thus it was that I found myself happily waffling along explaining how we computer nerds use additive primary colors of red, green, and blue (RBG) when mixing light to create our color displays. By comparison, printers tend to use subtractive primary colors of cyan, magenta, and yellow (CMY) when mixing inks.
Additive and subtractive color combinations.
Actually, forming black by mixing cyan, magenta, and yellow inks together is expensive and typically results in a "muddy" form of black, so printers typically augment these primary colors with the use of black ink. The result is referred to as CMYK, where the 'K' stands for "blacK" (we don't use 'B' to represent "black" because this could be mistakenly assumed to refer to "blue").
So there I was, as happy as a clam, when Gina said: "But surely that's not right. At school in our art class we were taught that the three subtractive primary colors for paints are red, yellow, and blue." By golly, she is right, that is what they taught us a school, isn't it?
In fact, if you cast your mind back to your school days, I'm sure that you'll remember something called a "color wheel" showing red, yellow, and blue (RYB) as the primary colors as shown below. Using these three primary colors as a starting point, we can generate three secondary hues: mixing red and yellow gives orange, yellow and blue gives green, and blue and red gives purple. Similarly, mixing the primary colors with their adjacent secondary colors results in six tertiary hues: red-orange, yellow-orange, yellow-green, blue-green, blue-purple, and red-purple.
A classical color wheel based on RYB primaries.
So how do we resolve this conundrum? Are the subtractive primary colors cyan, magenta, and yellow (CMY) or are they red, yellow, and blue (RYB)?
Actually, both options are true in their own way. By definition, the term "primary colors" refers to a collection of three (or more) colors that can be combined to form a range of additional colors. In reality, you can pretty much pick any three colors and call them "primary" colors, and this will be true on the basis that they are your primary colors. Mixing two of your primary colors together will result in a secondary color; mixing one of your primary colors with one of your secondary colors will result in a tertiary color, and so forth.
One example of a non-standard collection of primary colors was an early color photographic process known as Autochrome, which was invented circa 1903-1904 in France by the Lumiere brothers, Auguste and Louis. This process typically used orange, green, and violet as its primary colors.
In 1666, as part of his experiments with prisms, Sir Isaac Newton developed a circular diagram of colors (this is what we now call a "color wheel"). For one reason or another, theorists of that time decided that red, yellow, and blue were the best primary colors for pigments, and – even though we now know that red, yellow, and blue primaries cannot be used to mix all of the other colors – they have survived in color theory and art education to the present day.
The problem is that – the previous diagram notwithstanding – red, yellow, and blue are not well-spaced around a perceptually-uniform color wheel that embraces the entire spectrum of colors. This means that using red, yellow, and blue as primaries yields a relatively small gamut (where "gamut" means "a complete range or extent"), and it is impossible to mix them so as to achieve a wide range of colorful greens, cyans, and magentas. This is the reason why modern color photography and three-color printing processes employ cyan, magenta, and yellow as primaries, because these offer a much wider gamut of colors.
And as one last point of interest, it is common to refer to red, yellow, green, blue, white, and black as being the psychological primaries, because we subjectively and instinctively believe that these are the basis for all of the other colors. (If you are interested in reading further, check out the Primary Colors topic in my color vision paper.)
Questions? Comments? Feel free to email me – Clive "Max" Maxfield – at email@example.com). And, of course, if you haven't already done so, don't forget to Sign Up for our weekly Programmable Logic DesignLine Newsletter.