HSL (Hue, Saturation, Luminosity) allows us to describe meaningful relationships between colors. Give this brown color,
hsl(36, 73%, 10%), it's clear that if we desaturate 40 steps and lighten 70 steps we get
hsl(36, 33%, 80%), a cream color.
Look at that in hex,
#DDCFBB, or in rgb,
rgb(44, 29, 7) to
rgb(221, 207, 187), and the relationship between colors isn't evident in any meaningful way.
In graphics software I pick colors in HSB (Hue, Saturation, Brightness) because it feels more natural to work with than RGB or CMYK. Now, with CSS3 we can use HSL which is actually quite different than HSB. Without a decent HSL color picker, it's difficult to understand.
Pick a Hue from 0 to 360 and with saturation at 100 and luminosity at 50 and you'll have the purest form of that color. Reduce the saturation and you move towards gray. Increasing the luminosity moves you towards white, decreasing it moves you towards black.