A user is able to toggle field highlighting, which affects all active (non-read-only, non-hidden) fields. This is probably sufficient for what you want. What I was suggesting works if you just want to highlight certain fields and not others, but this doesn't sound like what you want to do.
The highlight color is determined by a user preference and defaults to a light blue color. Both the highlighting and the highlight color can be controlled via JavaScript, one of the few user preferences that can be:
http://livedocs.adobe.com/acrobat_sdk/9.1/Acrobat9_1_HTMLHelp/JS_API_A croJS.88.122.html
http://livedocs.adobe.com/acrobat_sdk/9.1/Acrobat9_1_HTMLHelp/JS_API_A croJS.88.123.html