Best viewed in modern browsers.
Opacity values range from 0 (transparent) to 1 (opaque).
Note that rgb values run from 0 - 255.
Note that hue is measured in degrees (from 0 - 360) and saturation and lightness values run from 0 - 100%.
In the last example (above) you will note that the opacity effects are actually additive. This might be counter-intuitive from other CSS effects you may have experienced.