Case color is done after the gun is in it's finished except for finishing. Would kind of defeat the purpose of hardening a gun if you were going to drill a bunch of holes through the hard finish.
While there are few certain tells there are a few. Sometimes the colors are just wrong. Case color is not one size fits all and people who do case work need to adjust their mix or system to duplicate factory colors. If the colors look wrong for a gun they probably are. Charcoal colors on a gun which was made when cyanide colors were done are a dead give away. A 1935 Fox and a Remington made Parker had cyanide colors. Refinished guns with charcoal colors may look nicer to many people but are period incorrect.
Look for wear under the case color. Look for telltale signs of shell wear on the face of the receiver and perfect case over it. Look for pitting on the receiver and case color over it. Look for high case color on a gun with tired finish overall.
There is no one sure fire tell for redone case in some guns. It is more of a feeling that you get when looking at the gun. And always remember to the seller the case color has never been redone even if there are finger prints under it. Do argue just smile and leave the gun in the hands of the seller when in doubt.
