Hex representation of a color with alpha channel?

Question

Is there a W3 or any other noteworthy standard on how to represent a color (including alpha channel) in hex format?

Is it #RGBA or #ARGB?

Answer 1

It looks like there is no hex alpha format: http://www.w3.org/TR/css3-color/

Anyway, if you use a CSS preprocessor like SASS then you can pass an hex to rgba: background:

rgba(#000, 0.5);

And the preprocessor just converts the hex code to rgb automatically.

Answer 2

In CSS 3, to quote from the spec, "there is no hexadecimal notation for an RGBA value" (see CSS Level 3 spec). Instead you can the use rgba() functional notation with decimals or percentages, e.g. rgba(255, 0, 0, 0.5) would be 50% transparent red. RGB channels are 0-255 or 0%-100%, alpha is 0-1.

In CSS 4*, you can specify the alpha channel using the 7th and 8th characters of an 8 digit hex colour, or 4th character of a 4 digit hex colour (see CSS Level 4 spec*)

As of July 2021, >93% of users can be expected to understand the #RGBA format

It has been supported since:

• Firefox 49, released Sept 2016 (Mozilla bug 567283).
• Safari 10, released Sept 2016.
• Chrome 62, released Oct 2017. For earlier versions you could enable experimental web features to use this syntax. See Chromium Issue 618472 and Webkit bug 150853.
• Opera 52, released March 2018 (or Opera 39 when experimental web features are enabled).
• Edge 79, released Jan 2020 (the first version of Edge based on Chromium)
• Samsung Internet 8.2, released Dec 2018
• Android P (your app must target Android P or newer)

It is not supported in:

• IE
• Original Edge (version numbers up to and including 18)
• Opera Mini
• UC Browser

Up to date browser support information is available on CanIUse.com

*Technically still in draft, but given the browser support this is unlikely to be changed.

Answer 3

I'm not sure if there is an official standard-

RGBA is the representation I've seen for Web Macromedia and others use ARGB

I believe that RGBA is the more common representation.

If it helps this is from W3 for CSS3
http://www.w3.org/TR/css3-color/#rgba-color

EDIT (Patrick): quote from the above W3 link

Unlike RGB values, there is no hexadecimal notation for an RGBA value

Answer 4

Chrome 52+ supports alpha hex:

background: #56ff0077;

For older browsers, you'll have to use:

background-color: rgba(255, 220, 0, 0.3);

Answer 5

You could try putting the hex color into the color picker of GIMP or photo shop to get the RGB value and then using the alpha value. eg. red is #FF0000 or rgb(255,0,0) if you want red with an alpha value of .5 then rgba(255,0,0,.5).

Maybe not exactly what you wanted but hopefully helps.