Colour theory encompasses colour mixing, harmony, and context. A colour wheel is a mixing tool consisting of primary, secondary, and tertiary colours arranged in a circle that looks like a wheel. The colour wheel is also divided into two halves of cool and warm colours.
The three primary colours are red, yellow, and blue. The three secondary colours are orange, green, and purple; they are derived from mixing two primaries. The six tertiary colours on the wheel are red-orange, yellow-orange, yellow-green, blue-green, blue-purple, and red-purple; they are produced by mixing one primary and one secondary colour.
Colour harmony simply means a structured, balanced, and orderly arrangement of colours that is pleasing to the eye. Colour harmony can be achieved using the tried and tested colour schemes or by looking at colour combinations in nature. Nature provides colour combinations that show different colours that work well together even if they depart from the popular colour schemes. Sometimes, unusual colour combinations in nature show up in perfect harmony.
The three most common colour schemes are analogous, complementary, and triadic colours. Analogous colours refer to any three colours that are side by side on the wheel. Complementary colours refer to any two directly opposing colours. Triadic refers to three colours that form a triangle on the wheel.