Is it good practice to make visited links different color from not visited links? Many browsers make visited links violet.
Or is it good to remove underline from visited links if not visited is underlined?
Or is there other better techniques?
It is better to have the color of the visited links different than the non-visited links. User actually believes on/requires Recognition rather than recall : one of the Human Computer Interaction principle.
If the color is different then it will be easier for the user to recognize which links he referred/visited so that he will not visit those links again if they are of no use.
Alternatively he can also use the links to visit if the links are useful as they will be recognized by different color.
It is indeed good practice to make visited links a different color. This helps a user to easily identify a visited link and identify if he need to go to the link or not. Sometimes he maybe looking to visit a new link when searching for something or sometimes, he need to revisit a link. So if there is a lot of links to choose from, then changing color of visited links is a very good idea.
Another attractive way would be to give images (like bulleting) and when a link is visited, you can change the image color. Its more attractive but the first method is simpler. You can give any color to visited or non-visited links but its better to keep it violet and blue since its a standard and new users wont get confused.
It's good practice to let users know where they've been, and by default this link is violet. But more importantly this link need to be different from unvisited links, and making it clear to the user which is which. Underlines on text have consequenses on readability, so don't use that - ever.
Reference: Top 10 Mistakes in Web Design (2011)
But if you want to use underlining to adress daltonians (achromate) users, it could be possible. The problem is that you can't remove the underline in visited links, since they wouldn't be noticed by those users. Maybe you can change from solid (_) to dotted (...) underlining? But to my knowledge there isn't a convention for this.
The main thing is still to diffentiate unvisited to visited links in a way users understand them.
Take a look at the The Definitive Guide To Styling Web Links article on Smashing Magazine.
There are tons of great suggestions and best practices there.
I went ahead and pulled out some of them to display here.
DON'T FORGET ABOUT LINKS