Mulches can be put down with or without a type of lining under them. Wood chips or shredded bark are often used on top of landscape fabric to achieve better weed control. Woven or layered landscape fabrics are made of various combinations of synthetics, such as plastics or vinyls, and allow air and water through while keeping light and weeds out. Do not use solid plastic sheeting because it creates a barrier to the movement of air and water in and out of the soil, and bark has a tendency to slide off it in heavy rains.

Dyed or colored wood mulches are made from waste materials, usually discarded shipping pallets and other waste wood. Sometimes, they contain tree harvest waste as well. The raw materials are chipped to a uniform consistency and then dyed a neutral color such as brown, redwood, or black.

Although some folks don't see the point of coloring the mulch, the people who sell it probably think that if it is dyed, it looks better than plain chopped up wood.

http://www.extension.umn.edu/yardandgarden/ygbriefs/h139mulch.html