Color temperature is a description of how warm or cool a light source appears, measured in Kelvin (K). The scale runs from very warm (1800K, candle flame) through warm-white (2700-3000K, incandescent bulb), neutral (4000-5000K), daylight (5500-6500K), and into cool blue (7000K+, overcast sky or shaded daylight). The number is technically based on the temperature a blackbody radiator would need to glow the corresponding color, but in practice it's just a useful one-number summary of light warmth.
In architectural rendering, color temperature is the single most important variable that determines whether a room feels warm and inviting or cool and clinical. Golden-hour direct sunlight is around 3000K — that's why golden-hour renders read so warm. Overcast daylight is around 6500-7500K — that's why overcast renders read so cool. Mixing different color temperatures within a single render (a warm lamp inside a cool-daylight room, for example) creates intentional visual tension and reads as 'lived in'.
AI rendering responds well to color-temperature direction in prompts. 'Warm 3000K light from the lamp' or 'cool 6500K daylight from the window' produces dramatically different output than the same prompt without temperature numbers. Even if you don't use exact Kelvin values, descriptive words like 'warm', 'neutral', 'cool', and 'bluish' all push the AI in the right direction.
See also