It seems not possible to highlight text in html or webarchive files.
I would like to be able to do this.
(1) May this feature be added some time in the future?
(2) Is it possible to edit the html-source via applescript so that i could highlight text in html files this way?
You can do that be yourself via System Preferences>Keyboard & Mouse>Shortcuts. Just add a shortcut, enter DTP as Application, the menu-name as it is displayed (the color from the submenu in your case) and enter the desired shortcut.
If you are on Leopard the shortcut should be ready to use as soon as you switch back. In older Systems you have to restart the application if I remember right.