CSS supports 140+ color names, HEX values, RGB values, RGBA values, HSL values, HSLA values, and opacity.
RGBA Colors
RGBA color values are an extension of RGB color values with an alpha channel – which specifies the opacity for a color.
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).
rgba(255, 0, 0, 0.2); rgba(255, 0, 0, 0.4); rgba(255, 0, 0, 0.6); rgba(255, 0, 0, 0.8);
The following example defines different RGBA colors:
Example
#p1 {background-color: rgba(255, 0, 0, 0.3);} /* red with opacity */
#p2 {background-color: rgba(0, 255, 0, 0.3);} /* green with opacity */
#p3 {background-color: rgba(0, 0, 255, 0.3);} /* blue with opacity */