Colour harmony
Also said as Colour scheme, Colour wheel, Complementary colours
Choosing colours by where they sit on the colour wheel. Neighbours (analogous) feel calm, opposites (complementary) feel tense and bright, and three spaced evenly (triadic) feel playful.
It gives you a reason for a colour, not only a taste. It also lets you plan a mood: a level can start analogous and turn complementary when things go wrong.
Say it
“Use a split between warm and cool: orange for the player and everything friendly, teal for the world, and nothing else above 30% saturation.”
Where you’ve seen it
Team Fortress 2 keys its two teams warm and cool: RED's bases are warm brown and rust, BLU's are cool steel and slate, so a glance at the walls tells you whose base you are in.
Not to be confused with palette
A palette is the list of colours you are allowed. Harmony is the relationship that makes them work, and a palette can have good or bad harmony.