It's not a line that's crossed. It's just the standard in Canada.

In Britain, aeroplanes are made of aluminium and they have tyres. The Ministry of Defence sends them out on manoeuvres in theatres of combat, where the pilots have generally excelled due to regular practice.

In America, airplanes are made of aluminum and they have tires. The Department of Defense sends them out on maneuvers in theaters of combat, where the pilots have generally exceled due to regular practise.

In Canada, airplanes are made of aluminum and they have tires. The Department of National Defence sends them out on manoeuvres in theatres of combat, where the pilots have generally excelled due to regular practice.

The line is drawn where ever the public puts it.

Generally, the correct spelling of a word is determined by those that use it. Canadians have used 'colour' for a long time. If enough people start using 'color', that will eventually be the correct spelling in Canada.