It is not necessary to model the pants since they were more like knickers and not visible beneath the robe.
The simplest way to create a Kohen robed in white is to use a white torso and legs. Although the "robe" look is lost when you reach the legs, this design offers the most flexibility in terms of posing the minifig for different parts of the sacrificial service:
You would think that the woven, purple belt could be easily accomplished by inserting a purple 1x2 plate between the torso and legs, but this cannot be done because the leg studs are too tall and will not accommodate a plate:
LEGO makes a turban headpiece for minifigs, part #40235:
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