Sounds normal, maybe casting temperature is a bit hot. The colour is just through refraction through the oxide layer. Without using assayed pure lead, with a certificate of purity or an electron microscope you will not be able to determine what you have. Watch for the metal going through a "mushy" stage between turning from solid to liquid and vice versa. A pure alloy will undergo a clear cut phase transition between solid and liquid phases, though some specific alloys, called eutectic, will do this too but these are very specific alloys. If you are loading for a revolver then anything harder than pure lead, roughly 8 BHN, will be a very hard to load. If for Minie bullets then simply try them, also pure lead will cast smaller; a .577 Minie cast for WW alloy comes out at .582" as opposed to .576" from the same mould.

The discolouring will have no impact on how the bullets shoot. The crystalline appearance is from the fact that, like all solid metals, lead is crystalline.