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Today I saw a post on Facebook that had this pun:

Clicked links turn purple because blue + "read" equals purple

Is there any specific reason why a link is blue and when it is clicked/read, it turns to purple?

Do these colors have any meaning?

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imao, the bright_blue should be faded_blue aka light_blue ... –  kmonsoor May 30 at 11:00
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This may be a pun because the participle 'read' sounds like 'red', and red+blue does yield purple. But if so, it's a pretty obscure reason, and I would never have thought of it. –  Kilian Foth May 30 at 14:34
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JUST IN CASE there is no joke to your post, you do know that most pedestrian posts, quotes and tidbits shared on the internet should be treated VERY SKEPTICALLY in regards to their truth. This statement is pretty cheeky and certainly not meant to be serious. However there is good reason for why they turn purple. Again, your post maybe intended to be funny. And you know this, but if not, don't believe everything you read... especially on facebook. :) –  Itumac May 30 at 17:47
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It's to confuse colorblind people like me. If I don't see a blue link for contrast, I can never know it's purple, unless the font is really large/thick. –  Vercas May 31 at 16:52
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Do these colors have any meaning? No! –  Awal Garg Jun 1 at 6:24

4 Answers 4

up vote 27 down vote accepted

Browsers render textlinks blue by default. Jeffrey Zeldman wrote an article stating Sir Tim Berners-Lee, inventor of the world wide web, reached it at random: http://alistapart.com/blog/post/why-are-links-blue

I can't find any information on why visited links are purple. It does however seems logical to me to choose a color close to the link color, preferably one colorblind people can distinct too.

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Note: colorblind people can't distinct followed/default (blue/purple) links easily. –  kaiser May 31 at 0:53
    
Both @Ruudt and @kaiser: There are several types of color-blindness: en.wikipedia.org/wiki/Color_blindness (most common sub-category being partial) –  IQAndreas May 31 at 6:19
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@kaiser I'm colorblind and I agree: Blue and purple are very hard for me to distinguish (unless they are next to each other and I can compare the brightness) –  Navin Jun 1 at 6:59

I can't say if this is the actual reason it was chosen, but a reason why purple is a good choice is that, other than red, it has the lowest relative luminocity of any hue.

So, a purple will tend to appear darker than an equivalent blue. Assuming the background is white and the text is black, and assuming we expect people to be more interested in pages they haven't already read, visited links will stand out less from a block of black text than unvisited links, but will still be noticeable.

Red isn't an ideal choice because in many cultures, it's associated with warnings and alarm.

Purple's low relative luminosity is consistent across the two most common forms of colour blindness (which are shown in this chart from wikipedia which shows them, somewhat ironically, with red and green lines... standard vision is barely visible behind red (deuteranopic, to green's protanopic) on the right in yellow):

enter image description here

Did they decide by studying a chart of relative luminosity? Probably not. But the fact purples appear dark could be the reason, for example a discussion could easily have gone like this:

"We need a colour for visited links. It should look darker so they stand out less."

"Dark blue?"

"No, people might not be able to tell if a link was dark or light blue if it was the only link on a page."

"How about purple? Purple is, you know, dark, it's not a bright colour like yellow or green."

"Yeah, looks good to me."

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What you say is true of differing the percieved luminosities of different spectral power distributions of different wavelengths (or in other words, red is dark and purple darker, as you say), but monitors don't produced equal power for each hue, nor even produce purple as a wavelength (they only produce red, green and blue wavelengths, but red and blue trigger the same cones as monochromatic purple). On monitors generally blue is darker than red is darker than purple is darker than green. This was even more true of 16-colour displays. Blue was already used for unvisited links though. –  Jon Hanna May 30 at 17:04

There can of course be more than one reason in favour of a decision, whether conscious or unconscious, yet more against bad alternatives (so they get rejected before we even see them) and hence I reckon the other answers here are also partially true. Indeed, quite likely so, since there's an answer here that talks about TimBL while I'm going to talk about Mosaic, which was independent from him.

One stated reason for Mosaic choosing the colour, is that 4-bit colour was still common, and so designs would need to consider how well they would look on such displays. Most such displays used two sets of colours, one of which was of the sequence:

0) Black 1) Blue 2) Red 3) Magenta 4) Green 5) cyan 6) yellow 7) white

(Note the fun way the mixing of colours and adding of numbers has the same results).

Then there would be another set of the same colours, but lighter (sometimes "light black" would mean grey, sometimes "dark white" would mean grey, sometimes both).

Of this sequence, the colours were darkest to lightest in that order (still true of the equivalents in sRGB, NTSC and other richer colour-spaces, though the exact proportions are generally different). Indeed the scheme was designed so you could depend on different relative shades when used with a monochrome display (black and white, black and green or black and orange).

Hence the darkest colour other than black (already used for normal text) that therefore offered the next-greatest contrast against a white or (as used by said Mosaic browser) grey background, was blue. Hence what would later (when attributes for describing colours where added to HTML) be called #0000FF was the standard link colour. (#000080 would also have been available, but not as distinguished from the black of unlinked text).

Red is well-known as a means of emphasis, it would be an appropriate choice for an active link (and was used as such, though I'm pretty sure not until later on, with Netscape Navigator 2.0 if memory serves, since before <frame> and javascript an active link didn't mean much as the page was going to unload anyway), but inadvisable for a visited link, which if anything should be de-emphasised compared to unvisited. After red, the next brightest colour, and so the next most appropriate for text other than blue and black was magenta, but the darker #800080 (purple) was sufficiently different to both blue and black to be distinguished from them, and again the darker choice and hence that which stood out best against a light background.

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It all depends on the CSS for the page. It may not always be purple. The CSS :visited selector is what controls it: http://www.w3schools.com/cssref/sel_visited.asp

example:

<style>
    a:visited {color:#00FF00;}   /* visited link */    
</style>
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8  
while this is true, this post was asking about the decision behind why the DEFAULT link is blue and the DEFAULT visited link is purple. An answer like that would be more appropriate for stackoverflow. this forum is more for the logic behind implementation decisions. –  Anindya Basu May 30 at 22:41
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It's also anachronistic; CSS wasn't published until 1996, and the first (very limited) support was that year, while the obsolete attributes for affecting colour that Netscape introduced weren't until around 1995. Colored links in set defaults were around since at least 1993. Of course, if it had all happened together, the design decisions behind the defaults would still be worth discussing. –  Jon Hanna May 31 at 13:52

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