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Quixotic said..
As I understand it, if neither boat is coming down on the other from behind (ie overtaking), then boat on port tack gives way to boat on starboard tack. If one boat is coming down on the other from behind (more than 22.5 degrees abaft of other's beam) then it must avoid whether it is on port or starboard tack. If neither can decide, then both must take whatever avoiding action is necessary and do so in a way that is clear to the other vessel as to what course alteration they are making, in good time. If necessary, they can signal their course alteration with noise (horn or whistle blasts) or light flashes (rule 34). Other than that, the colregs don't base right of way on a description of who can see which coloured light on the other boat - even tho' that may be how you spot the other boat at night.
I have read that the vessel on port tack should assume the down wind vessel is on starboard tack..
where I get confused is the down wind vessel classed as being upwind so have to give way or does his rights remain because the other vessel is on port tack.