A blue-collar worker is a member of the working class who performs manual labor. Blue-collar work may involve skilled or unskilled, manufacturing, mining, construction, mechanical, maintenance, technical installation and many other types of physical work. Blue-collar work is often paid hourly wage-labor, although some professionals may be paid by the project or salaried. There is a wide range of payscales for such work depending upon field of specialty and experience. White-collar workers typically perform work in an office environment and may involve sitting at a computer or desk. Service workers, or pink-collar workers, make up a third type of worker. Their labor is related to customer interaction, entertainment sales or other service-oriented work.(阅读以上材料,回答问题。) Question: What do blue-collar workers, white-collar workers and pink-collar workers refer to?