In the former fortress town of Naarden, the exterior woodwork of the military buildings from both the seventeenth and nineteenth centuries is still painted in a dark shade of green known as ‘monumentengroen’. The colour plays an important role in the buildings’ appearance. The owner of the fortifications, Stichting Monumenten Bezit (SMB), had doubts about the historical authenticity of this colour because previous colour research had revealed the presence of a lighter shade of green beneath the dark Monumentengroen. Accordingly, in 2018, in advance of a scheduled refurbishment of the exterior paintwork, another chromatic analysis was carried out in collaboration with Josefien Tegelaar of Josefien & Co. The firm investigated the colour finish on three external doors from the period 1872–1879. At the same time, SMB conducted supplementary archival research in the hope of unearthing a record of the earlier colour composition used on external doors and shutters. This article presents the results of the colour analysis and also shows that the colour used in military architecture was carefully considered and predetermined. Archival research showed that the exterior woodwork of military buildings in the Netherlands was painted in English Red until around 1860, after which the mandated colour changed to dark grey. Interestingly, the dark grey was only used for a very short time: during a major upgrade of the Naarden fortress between 1872 and 1879, the colour of the exterior woodwork changed from dark grey to blue-green. More information about the composition of the blue-green came to light while consulting the archives held by the Information Centre of the National Military Museum. The ‘Algemeene Voorwaarden voor de uitvoering van werken en leveringen voor den dienst der Genie’ (General Conditions for the execution of works and provision of supplies for the Fortifications Department, or AV), contain specifications regarding the construction of Dutch military buildings and provide conclusive evidence as to the prescribed materials and the manner of execution. The colours red, grey and green discovered during the archival and colour research, were also found in microscopic paint analysis of a paint sample from a watergate in the Oud Molen bastion. The microscopic analysis revealed that the green had been used more generally. It was made up of Prussian or Berlin blue, yellow and an addition such as Spanish green, which gave the green a bluish tinge. This, too, was specified by the AV. This colour research was conducted for the Naarden fort, but it can in fact be applied to all Dutch forts and fortress towns. As a result of the research results, the SMB decided to reinstate the historical blue-green colour on external doors and shutters of seventeenth- and nineteenth-century military buildings and is currently carrying out a phased change of colour in the fortress.