For a moment, at least, we will remove discussion about links from the two lines alotted in your typical learn-HTML-in-5-minutes tutorial, and admit that web authors actually put some thought -- ideally -- into their linking practices (hint: if you don't, start now). Gotta love that humble <a> tag. It's UI! It's design! And there's more than one way to do it!
The idea is not to make fresh links blue and old links purple, but to make fresh links look fresh and old links look old. The point, you see (and you'll see it again, trust me) is that we must communicate certain information. Usability rules work well as defaults, though if you know what you're doing you can break the rules with impunity. Careful choice of colors can often communicate better than dumb ol' blue and purple. Example: my dreams site, where the text is black, the links are red, and the visited links are gray. Red is a very visible, active sort of color, whereas gray looks subdued.
Whenever I see a good link color combination, the links appear at first glance eager to be clicked on, and after the diversion, worn out and in need of rest. Kind of like a puppy. I've also used, at times, the trick of making visited links the same color as the surrounding text so that they're more unobtrusive -- you can gloss over the name of a visited site without hearing it scream "click me! click me!"
So are there Do and Don't colors for links? I would venture that (unvisited) links of the same color as the text may be overlooked as underlining by some small proportion of the population. Blue/purple links, on the other hand, can look out-of-place on a webpage whose color scheme uses neither blue nor purple. For example, the Alfred University site, whose design includes pale yellow, olive green, and small amounts of purple -- none of which resemble the standard blue and purple that they use for their links.
In the same vein, what about the underline? Though unlinked, underlined text is inarguably bad (I know someone who once made all of his page titles blue and underlined ... sigh) what about links that aren't underlined? On the one hand, it looks trendy. On the other, it disables the automatic clicking mechanism in the user's brain that normally feeds off of underlines. As with coloration, though, context determines whether we can break the rule. If you can make text look like a link without underlining it -- hints might be to make it boldface and in a drastically different color from the main text -- go for it.
WPLUG uses this method, and can even get away with a color that's similar to the text because they've got context going for them: we encounter their funny format on words that can't be anything but links. First there are lists of things to see, and then there are email addresses, so that by the time we get to "Submit Feedback" at the bottom, we've figured out what the links look like. Another circumstance where I've seen un-underlined links work well is in a menu bar, where the only other interpretation of the words in the bar is that the author just likes to have big lists of colored words on every page. (See my organic site)
Hypertext was originally supposed to allow the user to click on any word they came across. Hypertext as we know it (ignoring the likes of click-anywhere dictionary software) is hardly this unobtrusive. A link just isn't a link unless it's bold or underlined, and in a very obvious color. Reading a sentence with irrelevant words hypertexted is distracting. Is it a necessary evil?
There's often no better way to provide background information; the distraction of the hypertext also serves the purpose of reminding the reader that the background is available. That's legitimate. That's what hypertext is for (the web model, at least).
But, despite the convenience of hypertext, people do like linearity. Achieving the right amount of linearity is important -- in an article like this one, a reader will likely want to read straight through, with only the briefest side trips (web-savvy readers will know they can open the links in a new window to avoid getting lost). Other situations have other requirements. A research paper can use links for citations, for instance, which are there more for academic integrity than as recommended reading.
But the situation to avoid -- unless, of course, you have a very good reason -- is the linking of more than the occasional or the necessary word. It makes the reader feel like Little Lulu in the cartoon where candy stores tempted her on her way to spend pocket change elsewhere. You wonder what you're missing and wish the distractions would go away, however appetizing they might be. An author who links too often is crying wolf. Just because every word in the sentence has an associated website does not mean that the reader wants to know about it.
If you need to fit in lots of hyperlinks, make them relevant. A reader will click out of curiosity: "The event reminded many of last year's snorfblatt." Thus, you can make the link useful by making it interesting. In other words, give people some clue as to why they should click on the word -- or, just as important, why they should not. In the example above, the reader knows that last year's snorfblatt is an event that was somehow similar to what they're reading about now. They will click if they want to know what the snorfblatt was, and will skip it if they don't care about such things. Simple enough.
The reader would also like to know where the link goes, in addition to what's on the other end of it. This document or a new document? This browser window or a new browser window? It's best to warn them, but how to do that is a very sticky question.
Because the HTML for within-page links involves a #, it's been suggested that in-page links be marked with a visible #. But this convention never caught on, so while it's a good idea, it's unusable. As kludgy as it might be, the best way to warn people is like this: my link (this page). Unless, of course, you're using a table of contents at the top of the page. Simply label it "table of contents" and people will get the idea.
New-window links are even worse. You can always do it like this: my link (new window) but, as above, it's inelegant. I believe that it's best not to open a new window at all, unless you warn the user somehow, or you use JavaScript (bad! don't rely on javascript*!) to make the window small enough that they realize what's happened. More often than not, users don't realize that a new window has been opened. And when they find out, they usually don't like it. A site that opens every link in a new window without good reason looks egotistical: "my site is so important that nobody should want to leave it."
Text links, simple as they may be, are something to consider carefully. When links form the basics of the web's UI, your linking practices are as important as the choice of icons a programmer puts in a toolbar: they must be well-selected and the user must be able to figure out how to use them. Remember that the point is not just to make your site look cool (otherwise you could hire a flash artist and be done with it) or just to follow somebody's generalized rules for usability. Considering the user gives you leeway in design: the message you send is more important than how it is sent.
* See the july 12 Daily News for one reason not to