Well, you are right sort of... You do NOT want to force the connectors onto the wrong posts, they can be really hard to get back off.
But if you use threaded connectors or can switch the hose, there is no problem pushing co2 into the "out" port of the keg.
Black Connector = Liquid Out Post
Grey Connector = Gas In Post
The only difference in operation is the length of the dip tube they connect to.
So, if you have a ball lock keg, you can use a "Black" connector connected to co2 attached to the "out" port of the keg.
Hope that makes sense.