In Safari you can create a custom style sheet and import it in Safari:
E.g. in Terminal enter:
echo 'a:visited { color: red !important; }' >> ~/Library/Safari/link.css
Then open Safari > Preferences > Advanced > Style Sheet > Other... and choose the links.css.
In older Chrome versions you can modify the file ~/Library/Application\ Support/Google/Chrome/Default/User\ StyleSheets/Custom.css (usually a 0 Bytes file) and add the same line:
echo 'a:visited { color: red !important; }' >> ~/Library/Application\ Support/Google/Chrome/Default/User\ StyleSheets/Custom.css
The support for custom stylesheets has been removed in newer Chrome versions. Here I recommend an extension: Stylist.