Are you talking about the 1/8 or 1/10th motors. When they first released the 1900kv and 2100kv 1/8 Trackstar motors the 2100's were silver and the 1900's were red. I actually still have one of their silver 2100's here. Then they went to all the same color for both kv ratings. Don't know the reasoning there except maybe to keep cost down a tad???

If your talking about the 1/10th motors I believe the difference in color is because they are two different type design motors from what I remember when I used to run them. The silvers looks like the older Reedy motors I believe.