Copied from the site's advice section.
Preparation and Installation guideUnderstanding the lampsCarefully remove the lamp from the protective vacuum form and examine it closely. You will see that the wire is ultra fine and appears bare, however, it is enamel insulated led wire (similar to that used on motor windings etc). You will also note that one wire is longer than the other. The long wire is the POSITIVE wire and should be connected to the blue wire if you are using a DCC decoder to control it. In the case of our 2 LED side/guards van lamp, there are two long positive wires and two shorter.

My lamps only have 3, is this an unposted upgrade?
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It's possibly an upgrade, with 2 feeds and 1 common return.
Maybe, probably a 2 colour LED
Are 2 the same length and 1 different?
If so, is the single longer or shorter?
This will then determine which is positive, etc.
Putting the feed to one of the matching length wires will give you 1 colour,
swapping to the other, same length wire will give you the other colour.
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