Really!!
If it were my car, i would be popping down and having a word with the tester and mentioning you are considering contacting vosa (the number is available on the direct gov website or filling in a vt17 online) and see what sort of reply that gets you.

It sounds like the tester has decided he just wanted to add this to the fail list with nothing more than his opinion, He might aswel have said "im failing it because the colour is offensive to me"!.