I know that the symbols on a Jaffa's forehead pertain to the System Lord to which they belong, but why do some Jaffa have black Tattooed symbols, while others have Gold symbols, while yet others have silver or multicolored symbols?
It indicates the Jaffa's rank. Your basic Jaffa soldier will have the symbol of his System Lord tattooed to his forehead in black ink. The First Prime (or ex-First Prime, for that matter) however, has a golden symbol instead to make him or her stand out from a group of regular Jaffa. Silver marks appear to indicate status of a Jaffa who is neither a First Prime nor a regular soldier.
The golden symbol is created by cutting the skin and pouring molten gold into the wound.