That has to do with the material. Different metals have different colors to them. Chromium tends to turn an inherent bluish, while nickel has a yellowish tint. Other alloying elements have their own colors too. It's not so much a color, but the tint that the shiny surface tends to have.
The method of manufacture has lots to do with it too. No matter what you do, a casting (like a cam core) has some inherent porosity. This isn't a bad thing, the pores hold oil better than say a billet part. Then there's the treating of the cam - many are phosphated (comp cams does a really aggressive phosphate) which can get down in the pores, and even once the cam is burnished and polished, that phosphate can show up as a colored hue. Either dark brown, gray, or white-ish depending on the type of phosphate and the oil used. Even unplated the pores can shift the hue of a part, or make it look 'dull' or 'hazy' even when polished.
All that said, that cam looks good.