Thanks for reaching out Alejandro! Bullet jackets are seldom made of pure copper. They are typically copper-zinc alloy. Any difference you notice in a bullet jacket's color is likely attributable to different ratios of copper and zinc in the alloy. For example, some jackets are 70% copper/30% zinc, whereas others are 90% copper/10% zinc. But whatever the copper-to-zinc ratio of an FMJ's gilding metal might be, you will notice no discernible difference in performance.