Editing web colors

Web colors are colors used in designing web pages, and the methods for describing and specifying those colors.

Authors of web pages have a variety of options available for specifying colors for elements of web documents. Colors may be specified as an RGB triplet in hexadecimal format (a hex triplet); they may also be specified according to their common English names in some cases. Often a color tool or other graphics software is used to generate color values.

The first versions of Mosaic and Netscape Navigator used the X11 color names as the basis for their color lists, as both started as X Window System applications.

Web colors have an unambiguous colorimetric definition, sRGB, which relates the chromaticities of a particular phosphor set, a given transfer curve, adaptive whitepoint, and viewing conditions. These have been chosen to be similar to many real-world monitors and viewing conditions, so that even without color management rendering is fairly close to the specified values. However, user agents vary in the fidelity with which they represent the specified colors. More advanced user agents use color management to provide better color fidelity; this is particularly important for Web-to-print applications.

Hex triplet
A hex triplet is a six-digit, three-byte hexadecimal number used in HTML, Cascading Style Sheets (CSS), Scalable Vector Graphics (SVG), and other computing applications, to represent colors. The bytes represent the red, green and blue components of the color. One byte represents a number in the range 00 to FF (in hexadecimal notation), or 0 to 255 in decimal notation. This represents the least (0) to the most (255) intensity of each of the color components. The hex triplet is formed by concatenating three bytes in hexadecimal notation, in the following order:
 * Byte 1: red value
 * Byte 2: green value
 * Byte 3: blue value

For example, consider the color where the red/green/blue values are decimal numbers: red=36, green=104, blue=160 (a greyish-blue color). The decimal numbers 36, 104 and 160 are 24, 68 and A0 respectively in hexadecimal notation. To obtain the hex triplet, we simply write the three hex bytes together without spaces, thus: 2468A0. If a byte is less than 10 (decimal) or 16 (hex) it must be represented with a leading zero to keep the number of digits in the triplet equal to six. For example, the decimal triplet 0,1,2 would be represented by the hex triplet 000102.

The number of colors that can be represented by this system is
 * $$256 \times 256 \times 256 = 16,777,216$$

An abbreviated, three (hexadecimal) digit form is sometimes used. Expanding this form to the six-digit form is as simple as doubling each digit: 09C becomes 0099CC. This allows each color value to cover its full range from 00 to FF.

Converting RGB to hexadecimal
Converting a decimal RGB value to a hexadecimal value is fairly straightforward. Decimal RGB values are usually given in the 0-255 range; if they are in the 0-1 range, multiply the value by 255 before conversion. This number divided by 16 (ignoring any remainder) gives us the first hexadecimal digit (between 0 and F, where the letters A to F represent the numbers 10 to 15. See hexadecimal for more details). The remainder, times 16 again, gives us the second digit. For instance the RGB value 201 divides into 12 groups of 16, thus the first digit is C. A remainder of 9 gives us the hexadecimal number C9. This process is repeated for each of the three color values. Most modern operating systems have a built-in calculator to perform conversions between number bases and this can also be done with some hand-held calculators. This task can also be carried out using various web resources.

HTML color names
The HTML 4.01 specification defines sixteen named colors, as follows (names are defined in this context to be case independent; the table is alphabetically ordered):

These 16 were also specified as sRGB and included in the HTML 3.0 specification which noted "These colors were originally picked as being the standard 16 colors supported with the Windows VGA palette."

X11 color names
In addition, a number of colors are defined by web browsers. A particular browser may not recognize all of these colors, but as of 2005 all modern general-use browsers support the full list. Many of these colors are from the list of X11 color names distributed with the X Window System. These colors were standardized by SVG 1.0, and are accepted by SVG Full user agents. They are not part of SVG Tiny.

Almost the exact same color names are used in .NET Framework, in the KnownColor and Color enumerations. The only color difference is DarkSeaGreen which is defined as 8F,BC,8B (instead of 8F,BC,8F).

The list of colors actually shipped with the X11 product varies between implementations, and clashes with certain of the HTML names such as green. Furthermore, X11 colors are defined as simple RGB (hence, no particular color), rather than sRGB. This means that the list of colors found in X11 (e.g. in /usr/lib/X11/rgb.txt) should not directly be used to choose colors for the web.

The list of web "X11 colors" from the CSS3 specification, along with their hexadecimal and decimal equivalents, is shown below, compare the alphabetical lists in the W3C standards.

Web-safe colors
Another set of 216 color values is commonly considered to be the "web-safe" color palette, developed at a time when many computer displays were only capable of displaying 256 colors. A set of colors was needed that could be shown without dithering on 256-color displays; the number 216 was chosen partly because computer operating systems customarily reserved sixteen to twenty colors for their own use; it was also selected because it allows exactly six shades each of red, green, and blue (6 &times; 6 &times; 6 = 216).

The list of colors is often presented as if it has special properties that render them immune to dithering. In fact, on 256-color displays applications can set a palette of any selection of colors that they choose, dithering the rest. These colors were chosen specifically because they matched the palettes selected by the then leading browser applications. Fortunately, there were not radically different palettes in use in different popular browsers.

"Web-safe" colors had a flaw in that, on systems such as X11 where the palette is shared between applications, smaller color cubes (5x5x5 or 4x4x4) were often allocated by browsers — thus, the "web safe" colors would actually dither on such systems. Better results were obtained by providing an image with a larger range of colors and allowing the browser to quantize the color space if needed, rather than suffer the quality loss of a double quantization.

As of 2007, personal computers typically have at least 16-bit color and usually 24-bit (TrueColor). Even mobile devices have at least 16-bit color, driven by the inclusion of cameras on cellphones. The use of "web-safe" colors has fallen into practical disuse, but persisted in culture.

The web-safe palette system persists as being the palette with the greatest number of distinct colors, where each color can be distinguished individually by human eyes. This led to the use of web-safe colors in anti-phishing systems.

The "web-safe" colors do not all have names, but each can be specified by an RGB triplet. Below are the values for the 6 shades of each color out of 256 possible color shades.

The following table shows all of the "web-safe" colors, underlining the really-safe colors. The lack of gamma correction means that the six desired intensities 0%, 20%, 40%, 60%, 80%, and 100% are displayed as 0%, 2%, 10%, 28%, 57%, and 100% on a Windows standard 2.5 gamma CRT or LCD, making most colors very dark. The intensities at the low end of the range, especially 0 and 3, are nearly indistinguishable from each other:

Color table
Note that in the table below, each color code listed is a short-hand for the RGB value; for example, code 609 is equivalent to RGB code 660099.

Safest web colors
Designers were often encouraged to stick to these 216 "web-safe" colors in their websites; however, 8-bit color displays were much more common when the 216-color palette was developed than they are now. David Lehn and Hadley Stern have since discovered that only 22 of the 216 colors in the web-safe palette are reliably displayed without inconsistent remapping on 16-bit computer displays. They called these 22 colors the "really safe" palette; it consists mainly of shades of green and yellow, as can be seen in the table above, where the "really safe" colors are underlined.

CSS colors
The Cascading Style Sheets language defines the same number of named colors as the HTML 4 spec, namely the 16 listed previously. Additionally, CSS 2.1 adds the 'orange' color name to the list:

CSS 2, SVG and CSS 2.1 also allow web authors to use so-called system colors, which are color names whose values are taken from the operating system. This enables web authors to style their content in line with the operating system of the user agent. As of early 2004, it appears that the CSS3 color module will once again drop these values, marking them deprecated, but this may change.

The developing CSS3 specification will also introduce HSL color space values to style sheets:

/* RGB model */ p { color: #F00 }      /* #rgb */ p { color: #FF0000 }     /* #rrggbb */ p { color: rgb(255,0,0) }  /* integer range 0 - 255 */ p { color: rgb(100%, 0%, 0%) } /* float range 0.0% - 100.0% */

/* RGB with alpha channel, added to CSS3 */ p { color: rgba(255,0,0,0.5) } /* 0.5 opacity, semi-transparent */

/* HSL model, added to CSS3 */ p { color: hsl(0, 100%, 50%) } /* red */ p { color: hsl(120, 100%, 50%) } /* green */ p { color: hsl(120, 100%, 25%) } /* light green */ p { color: hsl(120, 100%, 75%) } /* dark green */ p { color: hsl(120, 50%, 50%) } /* pastel green */

/* HSL model with alpha channel */ p { color: hsla(120, 100%, 50%, 1) } /* green */ p { color: hsla(120, 100%, 50%, 0.5) } /* semi-transparent green */ p { color: hsla(120, 100%, 50%, 0.1) } /* very transparent green */

Accessibility

 * Some browsers and devices do not support colors.
 * For blind and colorblind users, Web content depending on colors can be unusable or difficult to use.
 * Mixing legacy markup like bgcolor with modern CSS fails for legacy browsers not supporting CSS.
 * Both foreground and background color should be modified to avoid black on black effects.

Popular browsers show links as shades of blue. Dark background colors, such as blue or navy, do not display well for such links. Using CSS to modify link colors does not work with legacy browsers. For example, in Netscape 4.x support for CSS is frequently disabled for being rather paltry.