Broadly speaking, secondary colors are created by mixing two primary colors in a given color model. Therefore, it is not possible to say that a single set of secondary colors exist; secondary colors vary depending on the given color system. In order to grasp what secondary colors mean, one has to have a general understanding of color spaces and the primary colors in those given color spaces.
What is a Color Model?
A color model is the abstract system under which at least three colors are primaries. These primary colors are used to create secondary and tertiary colors. In the modern times, these color models have been developed for different purposes, each creating universal standards for its use. These models facilitate achieving the desired color universally. Various color models are used for print, screens, computers, fabrics and other purposes.
Secondary colors are basically made by mixing equal amounts of two primary colors of a specific color system. On a color wheel, they are in between two primary colors. Secondary colors depend on the color system/model they are created under. Based on the color model, they might be called additive or subtractive secondaries. Another aspect of secondary colors is that each of them share one of the primary colors within their color model.
Being the most traditional and historical color model, RYB is the abbreviation for Red, Yellow and Blue. This means that these three colors are the primaries in this model. Predating many aspects of color theory, the RYB is used particularly for painting. Its secondary colors are purple, orange and green, all created by mixing two of the primaries.
The secondaries in the RYB model are made by mixing the following primaries:
yellow + red = orange
red + blue = purple
blue + yellow = green
CMYK color model is the standard color system used in modern printing. CMYK is the abbreviation for the primary colors in this system: Cyan, magenta, yellow and key (black). As it is a subtractive color model, the secondaries of CMYK are also referred to as subtractive secondaries.
This because ink and paint absorb/remove some of the light wavelengths and reflect the remainder. Therefore, under this color model, mixing pigments will remove some light and thus produce a darker color. The secondary colors in the CMYK model are red, blue and green.
These secondaries are achieved as follows:
yellow + magenta = red
magenta + cyan = blue
cyan + yellow = green
Applied in light sources such as screens of smartphones, tablets, computers and TVs, the RGB model is also called the light model. For the human eye, the primary colors of light are red, green and blue. The RGB color model derives its name from the initials of these primary colors.
RGB is an additive color system. What this means is that adding light to a primary color in the system will produce a different and a brighter color. In the RGB color model, secondary and tertiary colors are brighter than the primary colors. In RGB, if all three primary colors are mixed in equal amounts, the end color would be white.
The secondary colors in the RGB color model are made as follows:
green + red =yellow
red + blue = magenta
blue + green = cyan