Pigments for polymers are generally in the form of a master batch, in which a concentrated dose of the pigment is blended with a relatively small amount of carrier resin. When the packaging material is produced, a metered amount of this master batch is blended with the base resin. Within the extruder, the master batch is blended with the base resin, and the pigment (and other additives) evenly dispersed. Ideally, the carrier resin in the master batch should be the same as the base resin. However, some master batch systems are available which are suited to a range of base polymers.