The Amana fan deck.

Three reasons compounding. Scale, colour intensifies as area increases, so a subtle chip becomes a saturated wall. Lighting, store lighting is nothing like your window light or your evening lamps. And surrounding colour, your floor, trim and furniture bounce their own tint onto the wall, which a chip in your hand never experiences. Large sample boards viewed in the actual room at several times of day are the only reliable method.

Three to five per room, no more. Beyond five, comparison gets harder rather than easier and most people end up more confused than when they started. Narrow the field with chips, then commit to proper large sample boards for the final few. Testing only two is usually too few, because undertone is a relative property that becomes visible only in comparison.

Light Reflectance Value runs from 0 to 100 and measures how much light a colour reflects. It is the most useful single number in colour selection and almost nobody uses it. Above 70 reads as light and bounces light around a room; 50 to 70 is mid-tone; below 50 is genuinely dark and will absorb light. For a basement or north-facing room, look for LRV above 60 to keep the space from feeling closed in.