RGBA color expressed using numbers.
background-color: rgba(255, 0, 0, 0.1);

RGBA color expressed using percentages.
background-color: rgba(0%, 100%, 0%, 0.1);

HSL color.
background-color: hsl(240, 100%, 90%);

HSLA color.
background-color: hsla(240, 100%, 90%, 0.25);