I usually do not try and tinker with the convergence too much, I usually set all the guns to the same range. Today though, I was going to try something new by varying convergences. Below are 3 examples that made me go

P47D-11: Blue and Yellow lines are secondary guns; white and green are primary guns (all .50 cals).
Me262: White is primary guns while green is secondary (all 30mm).
Mossi Mk VI: White is secondary (4/20mm) and primary (4/.30cal) has blue and green colors.
Spitfire Mk IX: White is secondary (2/20mm), primary (2/.30cal) has blue and green for colors.
I stopped there.
Is there supposed to be an assigned color for primary, secondary, and "tertiary" guns???