Colors can be defined in the Red-green-blue-alpha model (RGBa) using the rgba() functional notation. RGBa extends the RGB color model to include the alpha channel, allowing specification of the opacity of a color.
a means opacity: 0=transparent; 1=opaque;
rgba(255,0,0,0.1) /* 10% opaque red */ rgba(255,0,0,0.4) /* 40% opaque red */ rgba(255,0,0,0.7) /* 70% opaque red */ rgba(255,0,0, 1) /* full opaque red */