Most of us have seen grey heather, but do we all know that grey heather can never be 100% cotton (well, maybe some day that process will be invented). Anyway, for now, grey heather fabric is used by mixing at least two types of fiber.
I have done some research and compiled the following definition regarding heather:
A mixed fabric color is achieved by using different colors of fiber and mixing them together (a good example is a grey heather t-shirts). Black and white fiber mixed will combine to give grey heather fiber. Heather is blended fibers combined to create a multicolored effect. Heather fabric typically contains 3 fibers: cotton, polyester or rayon. Heather grey fabric is never 100% Cotton. Another fiber must always be added with the cotton in order to achieve the necessary color.
It is important to note that depending on the percent of cotton, the fabric can still be legally labeled 100% cotton. You will need to check the most current regulations to determine the percentage of cotton required to label a garment as 100% cotton. You can probably learn that information from the Federal Trade Commission. You can find a link to the FTC from the care label
page on Apparel Search or go directly to FTC.gov
Learn about textile fiber
such as cotton, rayon, and poly.
Learn more about fabric mills
I think that my definition can be improved. Hopefully one of you textile engineers or anyone else working for a mill can add more details. Please list your knowledge regarding heather in the comments section below.