Color Harmony - 101
Hello fellow photographers,
I had not fully understood color science since I started photography, and color science remained alien to me for at least 7-8 years into my photography journey. In fact I didn't even know what is the purpose of color circle until about a year ago.
How pathetic right? But it's never too late to learn!
Before we dive into color harmony, lets understand what is color circle.

Just Google and you'll find lots of examples on color wheel and theories.
A color circle, based on red, yellow and blue, is traditional in the field of art. Sir Isaac Newton developed the first circular diagram of colors in 1666. Since then, scientists and artists have studied and designed numerous variations of this concept. Differences of opinion about the validity of one format over another continue to provoke debate. In reality, any color circle or color wheel which presents a logically arranged sequence of pure hues has merit.
So, what is Color Harmony?
To put it simply, color harmony is the theory of combining colors in a fashion that is harmonious to the eye. In other words, what colors work well together.
It is the reason the Hulk wears purple pants. It is the reason the original X-Men had yellow and blue uniforms. It is the reason behind almost all color design decisions.
Types of Harmony
1. Complementary
This is the most basic harmony. It is a point opposite to the key color on the wheel. This “opposite” color is referred to as the complementary color and thus the direct harmony can also be called the complementary harmony.Virtually all color harmonies (except Analogous) are a variation of the direct harmony. It is the reason the wheel exists as opposed to a different kind of chart.

2. Split Complementary
Rather than the point opposite the key color on the wheel, the split complementary takes the two colors directly on either side of the complementary color. This allows for a nicer range of colors while still not deviating from the basic harmony between the key color and the complementary color.

3. Triadic Harmony
Also called Triadics or Triads. This refers to the color two spaces to either side of the key color’s complement. Essentially, with the triadic harmony, you are using three equally distanced colors on the color wheel. As such, you’re stretching the basic idea of color harmony and thus this harmony is best used with only touches of color.

4. Analogous Harmony
Also referred to as related colors,these are the colors directly on the left and right of your key color. They usually match up quite well and create a serene and comfortable design. While this color harmony can be pleasing to the eye, it can also come across as monotone. If you are going for a design that’s primarily one color, this is a good choice.

5. Monochromatic Harmony
Monochromatic color schemes are derived from a single base hue and extended using its shades, tones and tints. Tints are achieved by adding white and shades and tones are achieved by adding a darker color, grey or black. Monochromatic color schemes provide opportunities in art and visual communications design as they allow for a greater range of contrasting tones that can be used to attract attention, create focus and support legibility. The use of a monochromatic color provides a strong sense of visual cohesion and can help support communication objectives through the use of connotative color. The relative absence of hue contrast can be offset by variations in tone and the addition of texture.

Well, if you ask me, I usually refer to complementary or split complementary for my post processing works.
Hope this helps, and happy learning!
References: https://color.adobe.com/create, https://en.wikipedia.org/wiki/Monochromatic_color, https://zevendesign.com/color-harmony-hulk-wears-purple-pants/