What would happen if ground and neutral got mixed up in an outlet?
It will cause a lot of safety problems if you do that, but not immediately.  If anything else goes wrong in the circuit, it can have a variety of effects. One example is putting hot voltage on your grounds on that circuit, e.g. Every cover screw, chassis of lamps and machines, etc. 

It will immediately trip  GFCI/RCDs and some AFCIs, which might cause a frustrated user to remove them and go unprotected. 

Green insulated wire is ground wire.  Ground doesn't need to be insulated, but if it is, it'll be green or green/yellow. That applies internationally, it's the closest there is to a world standard.