On mods.tikiwiki.org (soon to have new theme) I have a star system there based upon users groups. Whilst Ive modifed the smarty routines to automatically display and select the correct one. Im guessing to change the colour of text or items your going to need to introduce some new CSS classes or new fields in users_users and do some serious template working to get them to display correctly.
With the forums I guess you could check the first character of each line for a > sign and work on the level and enclose it into a span tag. Shouldnt be too hard.