RGBA

RGBA color values are an extension of RGB color values with an alpha channel - which specifies the opacity of the object.

An RGBA color value is specified with: rgba(red, green, blue, alpha). The alpha parameter is a number between 0.0 (fully transparent) and 1.0 (fully opaque).

Example:
 webkit-box-shadow: 0 4px 0 0 rgba(65,156,192,1);
 moz-box-shadow: 0 4px 0 0 rgba(65,156,192,1);
 box-shadow: 0 4px 0 0 rgba(65,156,192,1);