Note that these are for boat wires. Individual equipment usually has just black and red wiring. For example, dark blue wire in a boat is the positive conductor for cabin lights (or cockpit lights in an open boat). But if you buy a replacement light fixture, it’s likely to come with a short trace of red and black wire. You would connect the red (fixture positive) from the fixture to the blue (lighting circuit positive) from the boat, through the fuse, breaker or panel to which it is wired.