What is the difference in color? (Black/White/Nickel)

The difference in color between the three models is primarily aesthetic. The black and white pieces are made from a high strength ABS. The nickel grinder starts with the same ABS piece and then it goes through an electroplating process where several thin layers of metal are coated over the plastic. The process is described in more detail below. 

Brushed Nickel Finish

The nickel grinders go through additional processes to achieve the brushed nickel finish. 

The first step is to take the ABS top and rings through an electroplating process. In the electroplating process a thin layer of copper coats the piece and then another process coats the pieces with a layer of nickel. The outcome is the original ABS pieces are evenly coated with a thin layer of shiny metal. 

To achieve the brush marks the pieces must then go through another process where each piece is brushed with an abrasive pad.

The electroplated pieces are slightly heavier and stronger than the original ABS pieces, but the main difference is an aesthetic one. All the internal components are the same as listed above.