In most cases it's just a few words, once or twice is a whole line; am coding interview responses by underlining in different colours and some fit into two themes so want to underline them twice if I can
Only way I can think that might work would be create a text box, remove the left, right and top sides, so you just leave the bottom side with a line, put your text into the text box and underline it as normal.
You can then change the colour of the text box and the underline independent of each other.
1.View/toolbars checkbox Drawing.
2. Far right of Draw applications has lines to insert.
3. Click on inserted line & choose color text, above Word page