Fool's gold looks vaguely goldish, but not all that much like gold. Among other things, fool's gold is a mineral, not a metal; if you hit it with a hammer, it shatters rather than getting dented.
If it is good tequila the barrel aging process ( the same thing that makes brown liqours brown) if it's cheap tequila carmel coloring, that's why gold tequilas are often anajos and blanco or white...