Green Linnets The 39th Foot, so called from the colour of their facings. Now the Dorsetshire, and the facings are white.
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Army reference - 39th Foot (presumably regiment) which had green facings on their uniforms. A number of regiments have been amalgamated over the years, so I guess that's what happened to the 39th Foot who had to give up their green facings when they became the Dorsetshire and adopt the new white ones.

As to Greenhorns, I would probably say that someone was 'still wet behind the ears'.