The gold color is tin oxide. When the surface of the molten solder is exposed to air it oxidizes and turns gold; sometimes the tin oxide is a purple-blue color. This is normal and can only be prevented by using an anti-oxidant powder or pellet. The discoloration is a cosmetic defect and in no way degrades the reliability of the solder. Kester offers the #5744 Dross eliminator for this purpose.
Source: Kester Solder