Tester pots are honest about colour — the pigment is the same as the tin. Where people go wrong is in how they test, and then they blame the paint.
Three mistakes wreck a sample test. First, painting a patch the size of a postcard — far too small to read a colour. Second, one thin coat, so the white wall grins through and the colour looks weaker than it'll ever be. Third, judging it wet, or in the wrong light. Paint dries darker and a colour shifts wildly between a north-facing morning and a warm evening lamp.
Do it properly: paint a patch at least A4-sized, ideally bigger, and give it two coats so you're seeing true colour. Better still, paint a sheet of white A2 card or lining paper and move it round the room — by the window, in the darkest corner, next to your sofa, against the skirting. View it at morning, midday and night before you commit.
The one genuine limitation is sheen. Most brands sell testers in a single matt or emulsion finish, even if you're buying the colour in eggshell or full gloss for trim. So judge a tester for colour, not for finish. A colour like Farrow & Ball Setting Plaster or Little Greene French Grey - Pale will read differently in dead-flat estate emulsion versus a modern emulsion or eggshell — more sheen bounces more light and lifts the tone slightly.
For the brands that make peel-and-stick samples — COAT and Lick both do proper self-adhesive swatches — those are brilliant for moving round a room without painting your walls, and they dry true. Dulux tester pots are widely available and reliable for what they are.
Practical advice: never pick a colour off a printed card or a screen, and never off a single small patch. Buy two or three testers, paint generous patches side by side, live with them for a couple of days, and trust the big patch over the tiny one. That's how you avoid a dodgy result and a wasted tin of topcoat.