Their is an additive added to make the plastic silver. In turn makes the plastic not as hard & strong as it could be. The black is more pure which is why it harder & stronger. It might have some additives but not as much as the silver does. I think in terms of raw plastic basically come in black or white. Additives are added to get the colors they want. That's why most stuff you buy that is plastic is either black or white. Their are other colors but not as strong. The reasons for two different colors who knows. Only someone from tamiya could answer why exactly it is like that. I hope you understand what I am talking about. I am not trying to lecture or make anyone feel weird. This is just my own take on the whole disscusion
Peace Aaron(W-W)