A wealthy and high-ranking darkeyes (of the first or second nahn) is addressed by the title Citizen. Darkborn is a term used to address a darkeyed person, though it is used very rarely. Darkborn is used only by a higher ranking lighteyed person addressing a darkeyed person, and its usage seems to imply either contempt or an obvious difference in status.
Changing eye color
Permanently changing one's birth eye color is generally not possible. However, two exceedingly rare methods are currently known to change one's eye color.
It is a well-known legend within Vorin society that if a man wins a Shardblade on the battlefield, he will become a lighteyes, regardless of his previous station in life. This would also be accompanied with a promotion to the fourth dahn for the Shardbearer. This legend, though a foundation of Vorin society, went untested for centuries, until Kaladin Stormblessed won a full set of Shards and gifted them to his lieutenant, Moash. Upon receiving the Shards, Moash's eyes did indeed lighten, from a dark brown to a tan color. It is unknown if this effect is permanent or only temporary while the holder is in possession of his Shards.
A second and even rarer method of becoming a lighteyes is to become either a Surgebinder or Knight Radiant. According to Skar, Kaladin's eyes changed while fighting to save Dalinar during the Battle of the Tower, although it was only for a short moment, as other men still recognize him as a darkeyes during the fight. During this battle, Kaladin only said up to the Second Ideal of the Immortal Words and did not recognize himself as a Knight Radiant. Later, during the Battle of Narak, after Kaladin says the Third Ideal of the Immortal Words and recognizes himself as a true Knight Radiant, his eyes permanently turn pale blue, lighter than any king.
It is speculated that the distinction between lighteyes and darkeyes this stems from the Knights Radiant. The Knights Radiant all used Surgebinding and Shards, which granted them light-colored eyes even if they didn't have them at birth. Because the weapons and magic that the Radiants wielded were so superior, the belief that light colored eyes being superior to dark colored ones came to be.
Hoid thinks that he should be amused by the import given to the color of one's eyes, but he isn't, since he has seen other arbitrary systems of governance; additionally, he recalls that there is a good reason why the belief in lighteyes came to be.
Those at the highest levels of nahn enjoyed relative comfort and wealth while those at the lowest level of nahn would be serfs restricted from being able to freely travel.
At the first and most prestigious nahn, one is even worthy of marrying into a lighteyed family. At the second nahn, one could also potentially marry into a lighteyed family. These two nahns are confirmed to be full citizens with the right of travel.
While those at the lowest level of nahn without the right of travel are essentially serfs tied to the land they are currently on, this is relatively rare. Many more people are at the slightly higher levels of nahn that include the right of travel.
Protections and privileges
There are some protections built into the system of nahn to prevent abuse by lighteyes.
The first is the the right of travel, which is assumed to be included with most nahns. To not possess that right would imply that one or one's ancestors had committed some wrongdoing. The right of travel allows a darkeyed person to leave a town being ruled poorly by the citylord. If the town experiences a substantial loss of persons, the citylord may be stripped of his position and be demoted in dahn.
The second known protection is exemption from military conscription for those who are of sufficient nahn and provide an essential function to a town. It is known that a second nahn surgeon and his only apprentice, also of second nahn, are exempt from military conscription.
Moving between ranks
Moving between ranks is a fairly fluid and common occurrence, except for the very highest ranks, which are harder to attain. The structure of the nahn system pushes people towards "stable ranks," which are ranks in which the child automatically inherits the same rank as their parents.
Joining the army is one way to permanently increase the nahn of a person and their children; for example, if a very low-ranked darkeye joins the army, he would most likely receive an immediate one nahn increase. Another way to increase one's nahn is to buy yourself up a rank, although this only works at the lower ranks of nahn.
There is also an extremely rare method to change ranks out of the nahn system and into the dahn system. By becoming a Shardbearer, regardless of method and whether one is darkeyed or not, one immediately raises his rank to the fourth dahn. This was accomplished for the first time in generations when Kaladin Stormblessed was granted a full set of Shards and, instead of claiming them for himself, gifted them to his subordinate Moash.
Inheritance of nahn
The nahn of a darkeyed child is influenced by the nahn that his parents hold. Generally, the child's nahn will be equivalent to the highest nahn that either of his parents hold. For example, a child from a union of a fourth nahn husband and fifth nahn wife would be fourth nahn.
However, for those of very high nahn, the children might slip down in rank until they hit a stable rank.
All darkeyes are not equal, however. There are ten different levels, or 'nahns', of darkeyes. For example, Kaladin's family is considered to be 'of the second nahn'.
A similar system is used to classify lighteyes. Instead of 'nahn', however, they use 'dahn'. A darkeyes of the first nahn can actually be more influential and wealthy than a lighteyes of the tenth dahn, although technically, the lighteyes is of a higher ranking. A darkeyes of one of the higher nahns is permitted to wed a lighteyes of a lower dahn.
It is strongly implied that it is a Vorin tradition. As such, countries that do not follow Vorinism generally don't have eye colour based classes. Sigzil points out numerous other systems of governance. Therefore, it stands to reason that it is not a universally recognized racial classification system.