HSL

Hue, saturation, lightness -- the same hue/saturation idea as HSV, but brightness measured as the midpoint between the RGB extremes.

HSL shares HSV's hue formula but defines $L=(C_{max}+C_{min})/2$ (the midpoint of the normalized RGB extremes) and $S=0$ if $\Delta=0$ else $\Delta/(1-|2L-1|)$ . A pure, fully-saturated primary has $L=50\%$ under this definition (not $100\%$ , as HSV's $V$ would give), since HSL reserves $L=100\%$ for white alone.