Color is a wavelength. Wavelengths can be measured and therefore the luminance of a color can be measured.
Luminance of a color is about how LIGHT or DARK it literally is; how much light it keeps and how much it is able to emit or bounce back into the room. Notice on the graphic how the lightness of the black on the far left is markedly less than the lightness of the neutral gray on the opposite end. When it comes to paint colors, the measure of luminance is available to you on paint chips or fandecks and it is called LRV. “Y” = light or luminance. See this article to better understand LRV and paint colors. LRV – What is Light Reflectance Value.