Color

 
#rgba {background: rgba(100%,0%,0%,0.4);}
#hsla {background: hsla(0,100%, 50%,1.0);}
#opacity { background: #000; opacity: .5;}

rgba(100%,0%,0%,0.4) produces 40% red

hsl(0,100%, 50%,1.0) produces 100% red.

black background set with opacity set at 50%