Daylight is not one colour. When the sun is low — early morning and late afternoon — its light travels through more of the atmosphere, which scatters away more of the short (blue) wavelengths and leaves the light noticeably warmer and more orange. Around the middle of the day the light is both brighter and cooler. A room therefore receives a moving sequence of light colours across the day, and a wall colour is really several slightly different colours depending on when you look at it.
Colours with a definite hue and moderate saturation stay recognisably themselves across the day, because the light's own colour shift is small relative to their own. Mid-toned colours also cope better than very pale ones, which have little of their own colour to hold on to.
Near-neutrals are the ones that move most: a beige can read pink at sunset and grey at noon, and a pale grey can pick up a distinctly warm cast in the evening. Rooms used mainly at one time of day — a bedroom used mornings, a sitting room used evenings — should be judged primarily in that light, not in the light that happens to be there when you are decorating.
Testing advice
Look at a sample at least three times: early, mid-afternoon, and after dark with the room's own lights on. If a colour only works at one of the three, it is not the right colour for a room used across the day. In an east-facing room, judge it in the morning; in a west-facing room, judge it in the late afternoon.
Lighting simulations and descriptions here are approximations, not exact physical predictions — see methodology.