Most exhausts have iron alloy, steel and galvanised somewhere..
ZINC is usually used for galvanising protection
Yellow zinc oxide can be the result where there is zinc, iron, and there is a catylist. (say salt /snow/water...)
So, where there is snow, and salted roads there are the ingredients required for yellow/green results.
The fact its been snowing, its dripping from the exhaust and garage have fed back they have seen this a few times - means this is very likely the cause.
(and when its cold you can get lots of condensation build up in the exhaust system).