Color Theory

Color wheel (A common way to understand how Color relate to each other)
Color systems (There is more than one way to represent Color in code, such as HSB, RGB, and Hex)
Warm vs. cool Colors (Warm color advance towards the viewer, cool Colors recede)
Color categories (Jewel, pastel, earth, neutral, and fluorescent tones, plus the shades (black and white))
Natural brightness (Some Colors like yellow are naturally brighter than other colors like violet)

Purpose

Uses for colour (e.g. meaning, separation, classification, etc.)
Cultural colour meaning (Cultures attach meaning to colour, and different cultures attach different meanings to colours)
Colour impact (Generally the less or more muted colours you use, the more impact other bright colours will have)
Denotation versus connotation (Denotative colours mean something specific, sometimes culturally and sometimes that the designer has decided. Connotations are meanings people associate with colours, often more than one per colour)
When to use light vs. dark palettes (Light mode is more approachable, mature. Dark mode is more cool, edgy, young)

Colour palettes (A selection of colours that work well together, usually picked for a specific project)