Primary Secondary and Tertiary Design Colors

Primary colors are red, green and blue.
They are called primary because the human vision system has three types of light detecting devices, most sensitive for the colors red, green and blue respectively.
Secondary colors are the colors that emerge when two primary colors are mixed. So there must be 3 secondary colors:
red + green = yellow
green + blue = cyan
red + blue = magenta or purple
There are also Tertiary colors and they arise when a primary color with one of the nearest secondary colors is mixed. So there are 6 tertiary colors:
red + yellow = orange
red + magenta = purplish red
green + yellow = yellow-green
green + cyan = blueish green
blue + cyan = greenish blue
blue + magenta = purplish blue