Hi Karl,
Great question. The color of the case denotes the capacity of the original battery. The gold battery is the most popular
Purple-396wh
Gold- 522wh
Blue-630wh
The higher the capacity the farther you can ride per charge. Turn your purple 396wh into a 522wh or even the long range 630wh while saving a pile of cash compared to a new battery. Before and after the rebuild process your battery is tested to assure the highest quality standards.