Although the NAT W track starts at COLOR, Oceanic's airspace does not start till just before coordinate 49°N 50°W (49N050W). This is the same for all the other eastbound NAT's.
Our aircraft has now just been handed off to the Gander Oceanic controller by the Gander domestic controller, this therefore means we must just be coming up to 49 North 50 West. When entering a track (or contacting any Oceanic controller for the first time) there is no special procedure, you just give a standard position report.
One thing to remember: Set your transponder to squawk mode C(S) / code 2000 latest 30 minutes after entering the NAT.