Allows colors to be defined via RGB along with an alpha (transparency) setting.
RGBA colors on W3C
The background color of this paragraph is defined using an RGBA setting of rgba(255, 43, 0, 0.2)
rgba(255, 43, 0, 0.2)
RGBa & HSLa CSS Generator for Internet Explorer
Devoth's HEX 2 RGBA Color Calculator
Working With RGBA Colour