(for fellow googlers, copied from duplicate question) This answer is outdated since text-decoration-color is now supported by most modern browsers.
You can do this via the following CSS rule as an example:
text-decoration-color:green
If this rule isn’t supported by an older browser, you can use the following solution:
Setting your word with a border-bottom:
a:link {
color: red;
text-decoration: none;
border-bottom: 1px solid blue;
}
a:hover {
border-bottom-color: green;
}