So you've been doing weddings for 10 years and only now, with today's cameras which deliver superclean images at stupidly high ISOs, you suddenly have an issue with noise? With 10 years of experience that's bonkers, as is your client management to be honest. If your client is wondering why the ceremony shots are in black-and-white then all you have to do is say "it's normal practice to provide ceremony and reception photographs in black-and-white whenever we encounter particularly low light conditions, or the effects of mixed interior lighting which can cause conflicting colour shifts within colour photographs. For this reason conversion to monochrome is deemed to produce the most pleasing results. For explanatory purposes I have attached a sample in colour".