Relative luminance isn't the same thing as how bright a color looks at a glance, it's a specifically weighted calculation where green contributes far more to the perceived brightness value than blue does, which is exactly why a pure, vivid blue can have surprisingly low luminance despite looking bold and eye-catching, a mismatch that trips people up guessing at contrast by eye rather than calculating it. This tool calculates a single color's relative luminance directly, predicting how it will read against both light and dark backgrounds before a second color even gets chosen. Useful for checking whether a specific color leans toward working better on a light or dark background before building a contrast pair around it, understanding why a vivid color reads as darker than expected in an actual contrast calculation, or getting a single color's luminance value to reason about ahead of picking what to pair it with.