So here is what proof I have. First is the assembly line picture. It is a low resolution, so it isn't super clear. I have a 232 MB picture that allows you to zoom in and really shows the white-ish color. The color isn't obvious until you compare to known colors like the aluminum intake and the valve cover. That jives with the person who says he saw a replacement manifold and was a white color.
The second pic is my manifold from the 220SE W128. I know from examining the entire surface, that it was top coated with Hi-temp silver at some point. That color can be seen as the lightest color in spots. But the base color below the silver with rust stains is clearly a white-ish color. Who knows exactly what shade, but the white lightning coating is close enough for me. From this, I think the conclusion can be drawn that it was a white color from the factory and certainly not left natural (cast iron) or black.