I know that nickel-silver (it's brass with less Zn and ~10 % Ni) bonds with stainless. But the colour is distinct. More yellow/gold than silver, but less so then brass. I've seen stainless filleted bikes (
Colossi and a French Co.) with what really looks like brass. No idea what the filler is. As mentioned there are thicker silvers appropriate for filleting. So it's possible!
(edit: Unter beat me to it. Maybe the polishing accentuates the difference between SS and filler and makes it look more yellow)