any dark opaque colors can not happen with gold or silver. The colors come from their colors at the atom level. Fuming is actually the first nanotechnology, being used by glass artists to turn the glass red or pink.
Explaining how this happens is for a scientist. Here is some help.
From what we know is that when we fume we are letting the gold stick to the glass, atom by atom. If an artist adds a lot of gold, you will see gold. Where there is just a little you will see pink (light red to a slight purple). When combined with silver you have the blue from the atoms of the silver that will help make purple or even green.
Yellow will not really ever happen, unless it is actually gold.
A super light silver fume will give a purple effect.