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 */