Aluminium wire is brown anodized to blend as much as possible with the tree being wired. It is soft and easy to apply to trees with a bark which damages easily such as azalea, maples and olives to name a few.
Copper is the traditional material used for wire and it has a much stronger holding power thanks to "work hardening." This enables you to use thinner and less wire to hold a branch in the position you bent it. The copper wire is annealed to soften it and is imported from Japan. Highly recommended for pines, junipers and other such species which have a thicker, more resilient bark.
Be sure to use an appropriately sized wire cutter for the thickness and material of wire being cut, or you may damage the tool.