I believe the Queen Mary is regularly used in films and TV dramas. The filmmakers probably had three choices - use a computer generated ship (film Britannic), or build a replica of a large liner (film Titanic), or simply use a cost-effective option and film the Queen Mary hotel at night and use her as a backdrop in the film. The director would have wanted to use a ship that was from the right time period but I doubt he cared very much about the peace time paint that was on the Queen Mary. Film directors quite often can only work with the tools they have got. Film budgets and production limitations factor out a lot of historical accuracy. Big budget multi-year film projects like Cameron's Titanic were riddled with problems and limitations which resulted in historical inaccuracies. It also depends on the specific goals and time restrictions of each production. They may have only had 1 available weekend to film the Queen Mary scene. They needed to hide modern buildings and members of the public out of the shot and decided to film late at night when it was empty. The paint that can be seen was probably the last thing they were worrying about at the time. When it finally came to the post-production and editing stage the director might have overlooked it, or he was willing to gamble that the general public would not notice the historical inaccuracy.
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