Transparency is a property outside the color itself, and it s also known as alpha component. You can t code transparency as pure RGB (which stands for red-green-blue channels), but you can use the RGBA notation, in which you define the color + alpha channel together:

color: rgba(255, 0, 0, 0.5); /* red at 50% opacity */

If you want "transparent", just set the last number to 0, regardless of the color. All of the following result in "transparent" even though the color part is set to 100% red, green and blue respectively:

color: rgba(255, 0, 0, 0); /* transparent */
color: rgba(0, 255, 0, 0); /* transparent */
color: rgba(0, 0, 255, 0); /* transparent */

There s also the HEXA notation (or RRGGBBAA) now supported on all major browsers, which is pretty much the same as RGBA but using hexadecimal notation instead of decimal:

color: #FF000080; /* red at 50% opacity */

Additionally, if you just want a transparent background, the simplest way to do it is:

background: transparent;

You can also play with opacity, although this might be a tad too much and have unwanted side effects in your case:

.half {
  opacity: 0.5;
  filter: alpha(opacity=50); /* needed to support IE, my sympathies if that s the case */
}

HEXA - #RRGGBBAA

There s a relatively new way of doing transparency, it s called HEXA (HEX + Alpha). It takes in 8 digits instead of 6. The last pair is Alpha. So the pattern of pairs is #RRGGBBAA. Having 4 digits also works: #RGBA

I am not sure about its browser support for now but, you can check the DRAFT Docs for more information.

§ 4.2. The RGB hexadecimal notations: #RRGGBB

The syntax of a <hex-color> is a <hash-token> token whose value consists of 3, 4, 6, or 8 hexadecimal digits. In other words, a hex color is written as a hash character, "#", followed by some number of digits 0-9 or letters a-f (the case of the letters doesn’t matter - #00ff00 is identical to #00FF00).

8 digits

The first 6 digits are interpreted identically to the 6-digit notation. The last pair of digits, interpreted as a hexadecimal number, specifies the alpha channel of the color, where 00 represents a fully transparent color and ff represent a fully opaque color.

Example 3
In other words, #0000ffcc represents the same color as rgba(0, 0, 100%, 80%) (a slightly-transparent blue).

4 digits

This is a shorter variant of the 8-digit notation, "expanded" in the same way as the 3-digit notation is. The first digit, interpreted as a hexadecimal number, specifies the red channel of the color, where 0 represents the minimum value and f represents the maximum. The next three digits represent the green, blue, and alpha channels, respectively.

For the most part, Chrome and Firefox have started supporting this:

The alpha channel defines the transparency value of a color, so any color is 100% transparent as long as the alpha value is 0. Typically this four-channel color type is known as RGBA.

You can specify RGBA in CSS like so:

div {
    background: rgba(200, 54, 54, 0.5); /* 50% transparent */
}

Note that not all browsers support RGBA, in which case you can specify a fallback:

div {
    background: rgb(200, 54, 54); /* fallback */
    background: rgba(200, 54, 54, 0.5); /* 50% transparent */
}

More information regarding browser support and workarounds can be found here.

You can use this conversion table: http://roselab.jhu.edu/~raj/MISC/hexdectxt.html

eg, if you want a transparency of 60%, you use 3C (hex equivalent).

This is usefull for IE background gradient transparency:

filter:progid:DXImageTransform.Microsoft.gradient(startColorstr=#3C545454, endColorstr=#3C545454);
-ms-filter: "progid:DXImageTransform.Microsoft.gradient(startColorstr=#3C545454, endColorstr=#3C545454)";

where startColorstr and endColorstr: 2 first characters are a hex value for transparency, and the six remaining are the hex color.

There are two common approaches for this: either reserve a specific color as being "transparent," in which case you cannot use that color in images without it appearing transparent, or define a fourth channel alongside red, green, and blue called "alpha" which indicates translucency/transparency.

Very simple: no color, no opacity:

rgba(0, 0, 0, 0);

Use following hexadecimal code for transparent text colour: #00FFFF00