The Riddler's Revenge (1998) was actually the first B&M with rails painted.
http://rcdb.com/ig470.htm?picture=16
I also don't believe B&M had a "no rails painted" rule, nor did they advise against it. Painted rails do not have an impact or effect the performance of a ride. It was purely an aesthetic thing, and it was just the way parks chose to have their rides painted then.