CSS hexadecimal RGBA?

I know you can write ...

background-color: #ff0000;

... if you want something that is red.

And you can write ...

background-color: rgba(255, 0, 0, 0.5);

... if you want something red and translucent.

Is there any terse way of writing partially transparent colors in hexadecimal? I want something like:

background-color: #ff000088; <--- the 88 is the alpha

... or ...

background-color: #ff0000 50%;

I am getting all my colors in hexadecimal, and having to convert them all to the decimal 0-255 scale is annoying.

#The CSS Color Module Level 4 will probably support 4 and 8-digit hexadecimal RGBA notation!

Three weeks ago (18th of December 2014) the CSS Color Module Level 4 editor's draft was submitted to the CSS W3C Working Group. Though in a state which is heavily susceptible to change, the current version of the document implies that in the somewhat near future CSS will support both the 4 and 8-digit hexadecimal RGBA notation.

Note: the following quote has irrelevant chunks cut out and the source may have been heavily modified by the time you read this (as mentioned above, it's an editor's draft and not a finalised document).
If things have heavily changed, please leave a comment letting me know so I can update this answer!

> #§ 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.

##What does this mean for the future of CSS colours?

This means that assuming this isn't completely removed from the Level 4 document, we'll soon be able to define our RGBA colours (or HSLA colours, if you're one of those guys) in hexadecimal format in browsers which support the Color Module Level 4's syntax.

###Example

elem {
    background: rgb(0, 0, 0);           /* RGB notation (no alpha). */
    background: #000;                   /* 3-digit hexadecimal notation (no alpha). */
    background: #000000;                /* 6-digit hexadecimal notation (no alpha). */
    background: rgba(0, 0, 0, 1.0);     /* RGBA notation. */

    /* The new 4 and 8-digit hexadecimal notation. */
    background: #0000;                  /* 4-digit hexadecimal notation. */
    background: #00000000;              /* 8-digit hexadecimal notation. */
}

##When will I be able to use this in my client-facing products?

All jokes aside: it's currently only the start of 2015, so these will not be supported in any browser for quite some time yet - even if your product is only designed to work on the most up-to-date of browsers you'll probably not be seeing this in action in a production browser any time soon.

View current browser support for #RRGGBBAA color notation

However, that said, the way CSS works means that we can actually start using these today! If you really want to start using them right now, as long as you add a fall back any non-supporting browsers will simply ignore the new properties until they are deemed valid:

figure {
  margin: 0;
  padding: 4px;
  
  /* Fall back (...to browsers which don't support alpha transparency). */
  background: #FEFE7F;
  color: #3F3FFE;
  
  /* Current 'modern' browser support. */
  background: rgba(255, 255, 0, 0.5);
  color: rgba(0, 0, 255, 0.75);
  
  /* Fall... foward? */
  background: #ffff007F; /* Or, less accurately, #ff08 */
  color: #0000ffbe;      /* Or #00fc */
}

<figure>Hello, world!</figure>

As long as you're viewing this answer on a browser which supports the background and color properties in CSS, the <figure> element in result of the above snippet will look very similar to this:

Using the most recent version of Chrome on Windows (v39.0.2171) to inspect our <figure> element, we'll see the following:

The 6-digit hexadecimal fall back is overridden by the rgba() values, and our 8-digit hexadecimal values are ignored as they are currently deemed invalid by Chrome's CSS parser. As soon as our browser supports these 8-digit values, these will override the rgba() ones.

UPDATE 2018-07-04: Firefox, Chrome and Safari are support this notation now, Edge still missing but will probably follow (https://caniuse.com/#feat=css-rrggbbaa).

I found the answer after posting the enhancement to the question. Sorry!

MS Excel helped!

simply add the Hex prefix to the hex colour value to add an alpha that has the equivalent opacity as the % value.

(in rbga the percentage opacity is expressed as a decimal as mentioned above)

Opacity %	255 Step		2 digit HEX prefix
0%	     	0.00			00
5%	    	12.75			0C
10%	    	25.50			19
15%    		38.25			26
20%    		51.00			33
25% 		63.75			3F
30% 		76.50			4C
35% 		89.25			59
40% 		102.00			66
45% 		114.75			72
50% 		127.50			7F
55% 		140.25			8C
60% 		153.00			99
65% 		165.75			A5
70% 		178.50			B2
75% 		191.25			BF
80% 		204.00			CC
85% 		216.75			D8
90% 		229.50			E5
95% 		242.25			F2
100%	    255.00			FF

#rrggbbaa notation is fully supported in Chrome 62+ and all other evergreen browsers:

background: #56ff0077;

RGB='#ffabcd';
A='0.5';
RGBA='('+parseInt(RGB.substring(1,3),16)+','+parseInt(RGB.substring(3,5),16)+','+parseInt(RGB.substring(5,7),16)+','+A+')';

Use red, green, blue to convert to RGBA:

background-color: rgba(red($color), green($color), blue($color), 0.2);

See here http://www.w3.org/TR/css3-color/#rgba-color

It is not possible, most probably because 0xFFFFFFFF is greater than the maximum value for 32bit integers

In Sass we can write:

background-color: rgba(#ff0000, 0.5);

as it was suggested in https://stackoverflow.com/questions/1419448/hex-representation-of-a-color-with-alpha-channel/13266076

I'm afraid that's not possible. the rgba format you know is the only one.

If you can use LESS, there is a fade function.

@my-opaque-color: #a438ab;
@my-semitransparent-color: fade(@my-opaque-color, 50%);

background-color:linear-gradient(to right,@my-opaque-color, @my-semitransparent-color); 
    
// result: 
background-color: linear-gradient(to right, #a438ab, rgba(164, 56, 171, 0.5));

If you have all your colors in HEX variables, you can use the following SCSS code:

First, copy this mapping from rgba - opacity values to hex codes:

    $opacity-to-hex: (
    0: '00',
    0.05: '0C',
    0.1: '19',
    0.15: '26',
    0.2: '33',
    0.25: '3F',
    0.3: '4C',
    0.35: '59',
    0.4: '66',
    0.45: '72',
    0.5: '7F',
    0.55: '8C',
    0.6: '99',
    0.65: 'A5',
    0.7: 'B2',
    0.75: 'BF',
    0.8: 'CC',
    0.85: 'D8',
    0.9: 'E5',
    0.95: 'F2',
    1: 'FF'
)

Then copy the following mixin that uses the mapping:

@mixin color-opacity($property, $color, $opacity) {
    #{$property}: unquote($color + map-get($opacity-to-hex, $opacity));
}

Last but not least, you can use this mixin with the mapped opacity on all color-properties, e.g. like this:

@include color-opacity('background-color', $your_hex_color, 0.8);
@include color-opacity('color', $your_hex_color, 0.5);

Charming Prince:

Only internet explorer allows the 4 byte hex color in the format of ARGB, where A is the Alpha channel. It can be used in gradient filters for example:

filter	: ~"progid:DXImageTransform.Microsoft.Gradient(GradientType=@{dir},startColorstr=@{color1},endColorstr=@{color2})";

Where dir can be: 1(horizontal) or 0(vertical) And the color strings can be hex colors(#FFAAD3) or argb hex colors(#88FFAAD3).

Well, different color notations is what you will have to learn.
Kuler gives you a better chance to find color and in multiple notations.
Hex is not different from RGB, FF = 255 and 00 = 0, but that's what you know. So in a way, you have to visualize it.
I use Hex, RGBA and RGB. Unless mass conversion is required, manually doing this will help you remember some odd 100 colors and their codes.
For mass conversion write some script like one given by Alarie. Have a blast with Colors.

how about using an :after pseudo-element?

#myDiv{
  position: relative;
  width: 128px;
  height: 128px;
  background-color: #ccc;
}

#myDiv:after{
  content: '';
  position: absolute;
  left: 0; 
  top: 0;
  width: 100%; 
  height: 100%;
  pointer-events: none;
  
  /* here you go */
  margin: -8px;
  border: solid 8px #00f;
  opacity: 0.2;
}

<div id="myDiv">hello world!</div>

why not use background-color: #ff0000; opacity: 0.5; filter: alpha(opacity=50); /* in IE */

if you target a color for a text or probably an element, this should be a lot easier to do.