The sniffer was painted sometime after 1988 from white to red (not grey). Here is a little history on the Ectomobile.
Here is the car sometime after the first film and before the second film, sitting in what looks to be Columbia's vehicle prop storage.
Now this next bit of history is weird. I don't know the order in which this happened for sure but I just know that it did. This is just what I think happened.
Either before or during production of Ghostbusters II, the car saw a few changes.
This is still the original car. Notice that the original logos have been removed but the sniffer is still white but more further weathered. Anyways, it sat in/on the storage/backlot like this for years (obviously having the original logos restored).
There are two theories on this next part. Some say that shots of the Ecto-1 in the second film were shot early on before the Ecto-1A came into being. The Ecto-1 saw a very brief refresh where the sniffer was painted completely red and GBII logos were adhered to the car doors.
However, if you were to go off the age of the rings on the raydome, one could surmise that this isn't the case. Also, the sniffer was never painted red. It just rusted out like the fanbox. It's also rumored that the Ecto-1 received a donor door from the Ecto-1A during its second restoration. That could be true but I don't know what to make of their being a GBII logo on the doors, when this is still pre-first restoration?
The car received its first restoration. This is where lots of little changes happened to the car with labels and paint schemes.
This is where new labels appeared on the sniffer body, which was converted back to white but leaving the top box part red (should be white). Then the car sat again for several more years until it got its (somewhat) full restoration which is how the car remains today.
Also to mention, the car does get routine upgrades. Notice the major rust on the bumper? This was after the major restoration. A year later and that rust was gone (sadly I can't say the same for the wood on the roof rack which is falling apart due to moisture and rain). When Sony does these upgrades, they do them on the cheap. I was out at Sony a few weeks before FanFest and they had maintenance repainting the 2016 Ecto with red spray paint. The red had sun faded so bad that it was almost a salmon color. Last time I saw the Ecto-1, there were bars of Irish Spring Soap all inside of the car to possibly mask the moldy smell the car can get. I guess that's a brief history of the Ecto-1 and its refreshes/restorations.