This information came from the W3C Specifications.

General Color

Colors can be specified by any of the following methods:

• RGB Hexidecimal notation color: #000 /* or #000000 */
• Constant color: black /* There is a list of valid values */
• RGB function notation: color: rgb(0, 0, 0) /* range is 0-255 */
• HSL function: color: hsl(120, 100%, 50%); /* (Hue angle, Saturation %, Lightness % */

Transparency is specified by:

color: rgba(0, 0, 0, 0.5) /* 50% opacity */

• RGB function notation: color: rgba(0, 0, 0, 0.5) /* or rgb(0%, 0%, 0%) */
• HSL function: color: hsla(120, 100%, 50%, 0.7); /* (Hue angle, Saturation %, Lightness %, opacity */

To make a background fully transparent, you could use the "transparent" value. Ex:

.clearBkgd {
	background-color: transparent;
}

Opacity

This is yet another method for making color and background color semi-transparent, but it can also be applied to DOM elements.

DOM elements (such as <div>, <p>, <h1>, <img>, etc.) can be given transparency by using the opacity property. Everything about that element (background, images, text...) will inherit the specified opacity. Ex:

.seventyPercent {
	opacity:0.7; /* for Webkit, Gecko, IE9+ */
	filter: alpha(opacity=70); /* for IE8 and earlier */
}

Color

The color name is not case-sensitive. "antiquewhite" = "AntiqueWhite"

Sort colors by Lightness or Hue.

Algorithm for converting between RGB and HSB