[steiner_j]

The colonia status survives in the name, that's indeed the main significance. There were of course hundreds of coloniae. Some similar, in that the status also gave the town its name today. E.g. Lincoln, England from "Lindum Colonia". Not every outpost in conquered territory was a colonia though. Some were oppida, while others were military outposts

In contrast to CCAA and Augusta Treverorum, Mogontiacum (Mainz) was not a colonia. It was primarily a military post. The main difference is that citizens of a colonia were in fact fully Roman with all the rights and duties. It was also part of a different province - Germania Superior, whereas CCAA was the centre of Germania Inferior.

Mogontiacum (Mainz) is further east than CCAA. One of it's purposes was to secure the Limes, but there was a comfortable buffer between the Limes and the town itself. Further north, the wilderness basically began when you crossed the Rhine at Colonia Ulpia Traiana (Xanten) or in fact CCAA...

You'll find the majority of Roman artifacts in an area roughly a third of the size of medieval Cologne, which itself is basically just the very city centre of Cologne today. However, a lot of digs were made along the roads leading out of Cologne since these were prominent burial sites for rich Romans. These roads follow the old layout until today...