Custom Google Visited Link Color v1.4.1
This is mostly for people who are colorblind and cannot tell the difference between visited and non-visited links.
How to set a custom color:
Go to the add-ons page and hit the Options button for Custom Google Visited Link Color.
At the bottom there will be an option to select a color, press the colored button on the right.
A color selector window will popup, select your desired color and hit OK.
Your color is now saved, all of googles visited links will now be the color you selected.