The introduction of red and green navigation lights for ships resulted in a number of collisions that were attributed to colour vision impairment. The UK Marine Department introduced screening tests for colour vision in the 1880s but when these were found to have limitations, the evidence on risk was reviewed and an adviser, Edridge Green with expertise in physiology, was engaged. Before his work was completed the Royal Society offered to recommend improved test methods. The Holmgren wool method was proposed by Abney, their leading expert and a pioneer of colour photography. It was used for the next 20 years but there was increasing evidence of poor validity and reproducability. This led to a test case supported by one of the marine officer's professional assocations' which was the subject of parliamentary questions involving Lloyd George and Churchill. The case was resolved by a trial nighttime voyage along the Thames and the Royal Society's advice became no longer sustainable when, despite failing the tests they supported practical colour discrimination was found to be perfect. This led to acrimony, to a scientifically well founded set of practical studies of distant night colour vision funded by the Marine Department and to the the introduction of a new form of testing using a lantern with coloured lights, an approach that remains standard up to the present day. Edridge Green pioneered this test but, despite the evidence, Abney and the Royal Society found it hard to accept the faulting of their position by good science and an early rival. This study exemplifies of the problems in the application of knowledge based on theoretical principles to a pressing practical problem. It also demonstrates the interactions between the scientific community, the political world and the emerging voice of employees in the early twentieth century. Finally the status, personal values and motivations of those offering expertise played a key part.