Metamerism is the reason a colour can play tricks on you. Put simply, it's when a paint colour shifts appearance under different light sources, or when two colours that match perfectly under one light no longer match under another.
It happens because colour isn't a fixed property of the paint — it's the result of pigments reflecting whatever wavelengths the light is throwing at them. Daylight is broad-spectrum and balanced. A cheap warm LED is heavy on yellow-red and short on blue. Halogen leans warm. Old fluorescent tubes are notoriously green and patchy. So a grey with a touch of blue in it — like Farrow & Ball Pavilion Gray — can read clean and cool by day, then turn flat and slightly lilac under a warm bulb at night. Greys and off-whites are the worst offenders because they're built from tiny amounts of multiple pigments, and those pigments respond differently to each part of the spectrum.
The classic real-world trap is matching new paint to existing trim or a fabric. Two beiges might look identical in the showroom's daylight tubes and then clash horribly under your sitting-room lamps. That's metamerism, and there's no fixing it after the fact — the mismatch is baked into the pigment recipes.
The most metameric colours are complex neutrals: think Farrow & Ball Cornforth White, Skimming Stone and Mole's Breath, all of which lean different ways depending on the light. Strong, saturated colours like Hague Blue are far more stable.
The fix is the same as for everything lighting-related: test in situ, in the actual light you'll live with. Paint two coats on a big bit of card, move it round the room, and look at it at the times of day you actually use the space — morning, dusk, and under your evening lamps. If a colour holds up across all of them, it's a keeper. And if you're matching to something existing, do that comparison under your home's bulbs, never the shop's.