Adding poison into food products is generally illegal everywhere and not at all likely for any company that wouldn't want to be sued into bankruptcy. Various bird food mixes are available with colored pellets which are usually intended to provide additional nutrients that aren't sufficiently present in the included seeds. It is less expensive to add synthetic sources of some nutrients than to include pricier seeds that would supply the nutrients in a natural form. In some cases such as vitamin D, there are no good natural / plant sources so it can only be added to food in a non-seed form. I have no idea how the practice of coloring the pellets began, nor why it continues, yet it is quite common for bird seed intended for certain types of birds such as parakeets.