A colour never looks exactly the same across a whole room. On a sunny wall, it might appear lighter, warmer, and more washed out. On a shaded wall or in a corner, the same paint can look deeper, cooler, or even like a slightly different hue.
If you only test in one spot—usually the brightest—you might love it there but hate it on the darker side once the whole room is painted. Testing patches on at least one bright and one shaded wall shows you the full range of how the colour behaves.
If you still like it in both conditions, it’s a safer choice. If it looks dull, dirty or strange in one area, you can adjust before committing to litres of paint and days of labour.
