[css] Hex colors: Numeric representation for "transparent"?

I am building a web CMS in which the user can choose colours for certain site elements. I would like to convert all colour values to hex to avoid any further formatting hassle ("rgb(x,y,z)" or named colours). I have found a good JS library for that.

The only thing that I can't get into hex is "transparent". I need this when explicitly declaring an element as transparent, which in my experience can be different from not defining any value at all.

Does anybody know whether this can be turned into some numeric form? Will I have to set up all processing instances to accept hex values or "transparent"? I can't think of any other way.

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:

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);

I was also trying for transparency - maybe you could try leaving blank the value of background e.g. something like

bgcolor=" "

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

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.