In COMBRITBOR HQ in Brunei in Nov 1962 it was decreed that for security reasons we were no longer to refer to aircraft types or airfields in clear, but to use codewords.
The airfields were to be colour coded - Black One, Brown Two etc.
We were using the ubiquitous Perspex and Chinagraph Maps and Wallboards. The Chinagraph colours we had were Red, White and Blue ( which were allocated to Kuching for the airfields in Sarawak ) as well as Black and Yellow. We needed more colours, by careful use of two colours, one on top of another, I was able to produce Green(Blue and Yellow), Orange(Red and Yellow), Brown(Black and Yellow) as well as Purple(Red and Blue). It looked a bit messy but it worked.