The go to preservative - primer for aluminium at that time for everybody was zinc chromate. Zinc chromate by itself in paint is the color called buttercup yellow. Frequently some black or a dark color is mixed in for a second coat of paint. Since it was primer there was no standard for the color. The Japaanese did not have a unified color system and colors varied frequently. Then paint was shipped in bags at mixed on site where painting was done (the US did this to). The solvent could be mineral spirits at the factory, but automotive gasoline, avation gasoline, diesel fuel or any other petroleum distilate on hand at bases. All of these can alter the color. Binding agents for the paint probably were suffering in quality because of shortages. Finally up until the mid 1950s color film had a problem with blue fadeing out. Colors you may see might have been different but lost their blue component. Also many of the records about such things were lost in 1945 when the second atomic bomb vaporized the IJN head quaters in Nagasaki.