Color for artists is most usefully broken into 3 elements of light: Hue, Saturation and Value. The default Color Picker in Photoshop however is most often broken into the 3 elements explained below, Hue, Saturation and Brightness.
Hue is the dominate wavelength of light, independent of purity and intensity. (e.g., Red, Orange, Yellow, Green, Cyan, Blue, Etc.)
The broader term Color is often colloquially misused in place of this element.
Note: This is measured in ° (Degrees) to correlate with a color wheel.
Saturation is a measure of the purity of light, independent of hue and intensity.
Colloquial examples: Warm when saturated, cool when unsaturated. Terminology that quickly becomes ambiguous due to relativity.
Use instead, Fully Saturated, Highly Saturated, Low in Saturation. Etc.
In Photoshop, Brightness is a measure of the intensity of the RGB pixels on your screen. This is not an accurate measure of the intensity of light. For clarity and future purpose we'll use the term Value to describe the intensity of light.
I'll explain this important difference in a future article.
Red's complement is not Green: The Color Wheel and accurate Complements.
A Wheel is a useful tool in understanding, analyzing and teaching Color. The simple geometry present allows it's user to intuitively determine the relationships between the three elements. Although it's true that as digital artists we don't have to worry about mixing our paints it's essential to understand these relationships in order to paint convincingly from imagination and to modify or enhance reference.
Many Color Wheels are incorrectly arranged so that Red is opposite and therefore complimentary to Green. The mixture above contains all hues between Red and Green with Yellow in the middle.
With a true compliment all hues are present and will create a neutral color. There are no other hues in this mixture outside the original two, it's a true complement.
In the Color Wheel below I've chosen Red, Green and Blue as my Primaries. Drawing a line between any two Primaries and finding the middle will result in our Secondaries; Cyan, Magenta and Yellow. The color opposite of another is that color's Complement, terminology that is often confused with compliment. To clarify, it is not a term to imply that these colors go well together, it instead refers to the completion of the Primary Triad.
e.g., Red's Complement is Cyan, we've already got one Primary - Red, therefore to complete the triad we'll need Blue and Green - which you've hopefully already noted are the components of Cyan.
Take a closer look at the graphic below and scroll down for a brief explanation of the text.
Left: Formula to determine Complements utilizing degrees. Below: Formula to determine Complements using RGB Values. Further Below: Quick reminder of Additive and Subtractive Primaries. Middle: A hue strip with Primaries, Secondaries and Tertiary hues, Degree of these hues to the left and RGB values to the right. Right: A RGB Primary Color Wheel.