Grey-collar refers to the balance of employed people not classified as white- or blue collar. It is used to refer to occupations that incorporate some of the elements of both blue- and white-collar, and generally are in between the two categories in terms of income-earning capability.
Grey-collar workers often have licenses, associate degrees or diplomas from a trade or technical school in a particular field. They are unlike blue-collar workers in that blue-collar workers can often be trained on the job within several weeks whereas grey-collar workers already have a specific skill set and require more specialized knowledge than their blue-collar counterparts.
The field which most recognizes the diversity between these two groups is that of human resources and the insurance industry. These different groups must be insured differently for liability as the potential for injury is different.
The least defined term of all seems to be grey collar, which can refer to those working well into their 60s, because they can't afford to retire, or to an underemployed white collar worker, such as someone with a bachelor's degree in English literature working as a customer service representative.