South-facing rooms (in the northern hemisphere) typically receive strong, warm direct sunlight for much of the day, which can intensify warm colours and wash out pale ones at peak brightness.
Hemisphere note: This describes typical Northern Hemisphere daylight. In the Southern Hemisphere, south-facing rooms typically get the cooler, indirect light described here for north-facing rooms — the physical mechanism is the same, but which compass direction gets the sun is reversed.
Cooler or more muted colours often read as more balanced in a south-facing room, because the light is already adding warmth of its own and does not need any more. This is one of the few situations where a cool grey or a true white works comfortably rather than reading as cold. Deeper colours also hold up unusually well here: there is enough light to reveal them properly, so a saturated colour that would be oppressive in a north-facing room can look rich instead.
Already-warm, saturated colours can feel overwhelming in direct sun for much of the day.
Testing advice
Check a sample at midday when direct sun is strongest, not only in the morning or evening.
Lighting simulations and descriptions here are approximations, not exact physical predictions — see methodology.