To change an NPC's traits, one will need to use an item that cannot be crafted in game called a Villager Editor. This will allow the player to completely customize the villager of their choice all the way down to their family tree and relationship with the player.

This makes roleplaying in non-survival worlds much, much easier if that's your cup of tea. 

 

In survival, however, there does not seem to be a way to adjust a villagers appearance, traits, or anything else aside from their relationship with the player.

However, there was once rumor many years ago of being able to use dyes to change villager clothes, though it is currently unknown if that still holds in the Reborn version of the mod.