Grey is never just grey, mate. Every grey is mixed from other pigments, and the way it's balanced gives it an undertone — green, blue, violet, taupe or red. The ones that look purple on your wall almost always have a violet or red undertone that you couldn't spot on the tin or a tiny chip.
Three things drag that undertone out:
Light direction. North-facing rooms get cool, blue-heavy daylight. That cool cast amplifies any violet or pink sitting in the mix, so a grey that looked neutral in the shop turns distinctly lilac on the wall.
Artificial bulbs. Warm white LEDs (2700K) push warm greys towards pink and mauve. If your grey already leans red, evening light makes it worse.
Surrounding colours. Greys are chameleons — put a cool grey next to warm flooring or a creamy ceiling and the contrast exaggerates the purple.
The classic culprits are the warm "greige" greys. Farrow & Ball Cornforth White is famous for flashing pink-violet in north light because of its red undertone — gorgeous in the right room, dodgy in the wrong one. Mole's Breath can go mauve under warm bulbs too.
Want a grey that stays grey? Go for one with a clear green or blue undertone instead. Pavilion Gray by F&B holds its cool grey beautifully without drifting purple. Pigeon has a green-grey backbone that reads steady. If you want something warmer but stable, Little Greene French Grey keeps its composure better than most greige.
Practical bit: never judge a grey from a chip or a screen. Get a proper sample pot, paint two coats on an offcut of lining paper, and move it round the room — by the window, in the darkest corner, under your actual evening lighting. Look at it morning, noon and night before you commit. The purple flash only shows itself in real conditions, and ten quid on a tester saves you repainting the whole room.