The yellow adventurer is from a western culture (eg: driik, sartola, islander), the grey cloaked adventurer is from an eastern culture (kaumo, reemi). the adventurers with the blue pants are from northern cultures (seal, kuikka, owl). i forget whether koivula and kiesse are east or west.
you can see the sprites in the truetile folder: npc-eadv.png, npc-wadv.png, npc-nadv.png
Similarly, the western woodsmen have yellow-brown outfits, the eastern ones have grey outfits, and the northern ones have red belts and blue pants.