LRV stands for light reflectance value. It is the share of visible light a painted surface bounces back instead of soaking up, on a scale from 0 to 100. A surface that absorbed every bit of light would score 0; one that reflected every bit would score 100. Neither exists as paint: even the brightest white sits below 100 and the deepest black sits above 0.
Think of it as the "lightness" half of a colour. The colour name tells you the hue (green, clay, grey-blue); the LRV tells you how pale or deep it is. Two greens can be the same hue and still be far apart, one at 10 and one at 60.
Makers measure LRV on the flat paint colour with an instrument, so it is a fair way to compare one colour with another. It is not a promise about how your room will look, because that also depends on the window, the lamps, the floor and the other things in the room.
Walls and ceiling are the biggest surfaces you own. Daylight from the window, and lamp light after dark, bounces from surface to surface several times before it fades out. A pale wall sends most of that light back into the room. A deep wall keeps a good share of it each time it bounces.
So dark walls do cost you some light, and you notice it most in the corners furthest from the window. That is not a mistake. Many people choose a deep colour for a snug room on purpose, because a room that holds the light close feels enclosed in a calm way. The aim is to choose knowingly: expect to want a lamp or two switched on earlier in the evening, and expect the colour to shift through the day.
The ceiling counts too. It is often the surface that bounces light across the whole room, which is why a pale ceiling can keep a darker-walled room from feeling heavy.
Move the wall slider from deep to light and watch the room. The number below it is a rough, simplified estimate of how much of the room's light is bounced around compared with the same room painted entirely white. It is for building a feel for the idea, not a lighting calculation.
Compare inside one brand's range. Each maker measures its own colours, and many list the LRV on the colour card or technical data sheet. If you are torn between two colours from the same range, the lower number is the deeper one.
Match it to the light you have. As a rough rule, a room that gets little daylight, such as a north-facing one, gets more help from a higher number, while a room with a big bright window can carry a lower one. A large open space usually stays open with a higher number.
Treat it as one clue, not a verdict. It tells you nothing about warmth or undertone, and a moderate number can still feel bold if the colour is strong. Screens are a poor judge too, so paint a large sample on a board or sheet of lining paper that you can move around the walls, and look at it in the morning and in the evening. Our guide to how paint changes through the day shows why.
You do not have to repaint. If you rent, check what your lease allows first. A darker room can feel warm rather than dim with a lamp or two at different heights, and light-coloured textiles and a mirror placed to catch the window do some of the same work as paint.