Why would you only want the underline?
But within the "Color in online list" option, using SMF 2.0.2 (and I assume later), you can use;border-style:solid;border-width:0 0 2px 0;border-color:green
in the field. Replace "green" with any color (or #rrggbb/#rgb for more specific options).
[img=http://i.imgur.com/CiGPOWI.png]http://Image.[/img]
Tested in Chrome 29, FF 26, and IE 8.