Seems reasonable to me, Lark - firstly, it saves confusion when the pax are confronted with an aircraft in a different livery, and second, there are some operators who's aircraft I wouldn't want to fly, and others will have similar feelings about some operators, so it would enable a refund and alternative arrangements to be made in advance if that were the case; if, for example, a Lion Air was on the tarmac for your wet leased flight, would you be quite so happy to board it?