I’m going to take a stab at it, in spite of never having worked with
gold-filled wire. What seems to be happening is that you’re getting
both the gold and the filler, some base metal, mixed up at the end of
the wire. Instead of gold, or the base metal, you’re getting an alloy
of the two, and it has different properties than either one would
have, separately. It melts differently, solidifies differently, and
is probably a different color as well.
That’s just what it’s doing, not how to fix it. To get a nice
smooth ball on the end of the wire, I think you’re going to have to
add some solder to cover up the base metal. Melt it onto the end
after you’ve balled it, I guess, or at the same time… I have no
idea which would be better, so you’ll have to experiment a bit, but
I’m sure that if you pick some solder that’s the right color it will
turn out great.
Heck, try contrasting colors and pretend you meant to do it that way
from the outset – I bet it would look great, and that way there’s no
subtle change in color that people can point to and question.
The ultimate fix, though, is to use solid wire instead of
gold-filled wire, and charge enough to make it worth it.