Union dye is also called as All-Purpose Dye. Unions are materials made of two or more different kinds of fibers, such as cotton and wool or viscose and acetate rayon. One means of dyeing unions is to dye each kind of fiber separately with the appropriate kind of dye and then to weave the colored yarns into cloth.

Union dye or All purpose dye is most useful when coloring a blend of protein fiber or nylon with a cellulose fiber such as cotton or rayon. Approximately the same color is dyed for both the fibers at the same time. Few examples of such blends are linsey-woolsey, Nycott (until it is not treated with Teflon, which makes it undyeable), and any cotton or rayon garment with nylon lace trim.

However, when dyeing a pure fiber of any kind or a mixture of a pure fibers with non-dyeable synthetics such as polyester (which require entirely different dyes), the mixture of dyes is considered as a waste of dye and money. At the time of dyeing pure cotton, the acid dye brightens the dyed items up only until it is washed for the first time, where all of the acid dyes disappear into the sewage system. Conversely, dyeing of wool or nylon alone, the direct dye is wasted, and ends up down the drain. Then why one will waste money on dye that won't even attach to your fiber? It makes more sense to buy pure direct dye (or, even better, fiber reactive dye), if the person dyeing cotton, and to buy pure acid dye if the person is dyeing wool or nylon. So, sometimes it becomes relatively expensive.

Use Union Dyes if the person wants to use a single step to dye a garment that comprises of a cellulose fiber, such as cotton or rayon, with either wool, another animal fiber, or nylon. Otherwise, if possible then we should avoid All purpose dye, in favor of a specific Acid or Direct or Fiber reactive dye that specifically matches with the person's fiber and meets his/her needs.