Light changes from wall to wall. A colour that looks perfect on the wall opposite the window might look dull or too intense on the wall next to it, where shadows fall differently. If you test paint in only one place, you’re judging it under one specific lighting condition.
By painting small patches on at least two or three walls – one with direct light, one in shade, maybe one near a corner – you see how stable the colour feels around the room. You might find it goes greener in shadow or looks much darker in a corner than you like.
That broader view helps you avoid surprises after a full paint job. You’re choosing a colour for the whole room, not just for that one bright patch.
