I do hope you are joking!
any form of product may attack the belt fibers and lead to them not doing there job in an accident.
If you must , you could paint the buckles etc as a highlighting idea?
Mmmmmm yeah I was kind of half joking.
I guess my dream for red seatbelts has gone out of the window now :-(
Just want to say thanks to all those who posted above! Just shows the power of a forum because I would have probably just gone out and done it, and wouldn't have thought about telling the insurance company! Obviously now, its so clear as to why they should know and the implications that it could have!