Lightness

HSL stands for hue, saturation and lightness. At 0% lightness the result is black; at 100% it is white, whatever the hue setting.

With saturation held at 100%, the blue example is vivid at 50% lightness. Below that it darkens toward black, and above that it becomes paler toward white.

HSL lightness is a color-model coordinate, not a reliable measure of how bright two different hues look to people. Check contrast separately.