I always thought the song referred to yellow girls in the context of WW1 British munition workers. They took on a yellow colour from the chemicals involved in the explosives. They were the first working group of women who were comparatively well paid and so had an unheard of degree of independence. The dangerous nature of their work also gave them a more carefree attitude to life. All of which earned them a racey reputation. I realise if this is true it makes the song quite recent when it was collected in 1914. Any thoughts?