This comparison table for IJAAF paint colour standard # 21 (green colour - midori iro 緑色) with hobby paints for the classic "Nakajima Dark Green" or "IJA Green" (sic), was prepared from measurements taken as a piece of consultancy work but might prove of wider interest and use to modellers - although somewhat pre-empting a much more detailed study. The original paint colours were measured in L*a*b* using a Hunter Lab 45°/0° Miniscan EZ Spectrophotometer and the analysis comparisons calculated using colour translation and analysis software. Difference quantifications were calculated using the DE2000 formula. This formula is recommended by the Commission International de l’Éclairage (CIE). The lower the number the closer the match - a difference calculation of 2.0 or less usually indicates a close match - but the figure is drawn from cumulative values so the direction of shift must be quantified by description and/or observation. Whilst wartime paint batch differences of up to (and even beyond) 5 were not unusual, for precise matching and visualisation purposes across standards any calculation more than 3 can be considered of limited value. The full working table that provides the measured and calculated L*a*b*, sRGB, actual Munsell, closest Munsell, closest FS595b, closest RAL values is available on request.
The most obvious conclusion is the lack of sufficient green chroma in many of the hobby paints purporting to be IJA Dark or Army Greens - most being generally more greyish or olive and somewhat darker. Although # 21 has the appearance of quite a bright green - it certainly has a strong green chroma - its olive characteristic is subtle but nevertheless present and can be better appreciated in juxtaposition against true bright or viridian (bluish) greens.
The Humbrol A3 and Thorpe A2 colours were quite possibly based on samples of IJAAF # 7 (the late war olive brown - Ohryoku nana go shoku 黄緑七号色) rather than # 21, but it is apparent that for many years the two colours have been conflated in references to produce something of a hybrid. However, at least two Japanese veterans have mentioned the dark green colour on early Hayabusa aircraft turning "reddish" with exposure - more towards olive or brown - which is a characteristic of chrome green pigment. Although chrome green pigment was often referred to and used interchangeably for chromium oxide (green) pigment during the war years it is an "artificial" green made from chrome yellow and prussian blue pigments. Over time the chrome yellow attacks the blue pigment and decomposes it, shifting the colour appearance towards the yellow pigment and any reddish or brown pigments contained in the mix. This chemical phenomena has been recorded even on quite recently applied military green paints where chrome green has substituted for chromium oxide (green). British and German technical documents from the 1939-45 period make it apparent that the distinction between the two pigments was confused (or their interchangeability accepted), for example by the Ministry of Aircraft Production (MAP), aircraft manufacturers and even paint manufacturers. There was a world wide shortage of chromium oxide (green) during the war so it would be no surprise if the interchangeability of the two pigments occurred more widely. The significant aspect for modellers is that camouflage greens made with these pigments degrade in different directions. Chromium oxide (green) pigments have been tested to show a gradual deterioration (over relatively long periods of exposure) towards a more "pea green" appearance.
Thorpe's A3 Medium Green is a lighter, brighter medium green that might have been based on degraded samples of paint made with chromium oxide (green). However, the Japanese paint colour standard 1-5 from the February 1945 document 8609 reconciliation of IJN and IJAAF paint standards, which replaced # 21 and was described as "grass colour" (kusa iro -草色), is also quite a bright looking and lighter green which was reported to approximate Munsell 2.5 G (Green) 4/4 by one Japanese researcher in 2005. However, a facsimile chip for colour 1-5 provided by the Koku Fan magazine editor and assessed in 1975 was compared to approximately 7.5 GY (Green Yellow) 3/4 or FS 34128 as shown above. The IJN colour D4 was also replaced by 1-5 so there might have been some compromise over its agreed appearance.
Image credits: Table and rendered colour chips © 2013 Straggler