Most people test paint wrong, and it costs them. The dodgy bit is painting a small patch straight onto the wall: you get a thin, patchy smear sitting over your old colour, viewed in one spot, in one light. That tells you almost nothing.
Do it properly instead. Get yourself some peel-and-stick sample sheets if your brand sells them — Lick and COAT both do brilliant peel-off swatches that save a lot of mess. If you'd rather use a pot, paint two coats onto a large sheet of white A2 card or lining paper, at least an arm's-length square. Two coats matters because most colours only show their true depth on the second — one coat of something like Farrow & Ball Setting Plaster or Cornforth White reads weaker and patchier than it actually is.
Then move the card around. Tape it to different walls. A north-facing wall pulls everything cooler and greyer; a south-facing wall warms it up. Hold it next to your skirting, your sofa, your flooring — colour never lives in isolation. Cornforth White is the classic cautionary tale: it can throw mauve or pink against the wrong neighbour.
Look at it across the day — morning daylight, harsh midday, and crucially under your actual evening lighting with the lamps on. Warm bulbs shift cool greys like Pavilion Gray noticeably. A colour you loved at 11am can turn on you at 8pm.
Never judge against a freshly painted white wall — the contrast tricks your eye into reading the sample darker than it is. Test against your existing wall colour, or prop the card so it's free-standing.
Give it at least 48 hours. Quick decisions on paint are how you end up repainting. And test the finish too, not just the colour — an estate emulsion and a modern matt of the very same shade reflect light completely differently. Buy the right testers, be patient, and you'll get it sorted first time.