Hey!
The underline code is handled a custom event - please check out the underline/strikethrough example scene to see how that works! (It's done this way so underlines can be drawn in any way without needing to be rigidly built-in. This method would let someone make a zigzag underline with little modification, for instance)
Basically, a custom event like <e2=underline>This text is underlined!</e2> is used to draw an underline under your text. The code I included lets you change colour per-mesh, so you should already be able to change the colour this way! Check out the components atached to the STM object in that scene, and that should explain it well!
-Kai