Sylvari

The sylvari, also known as the changelings, are a humanoid species native to the volcanic island chain of Kilai'i. Their common epithet is owed to their peculiar supernatural ability to mimic the appearance of other races. It is unknown how the species received this ability, nor what the true nature of the ability is. Extensive study has concluded that the changelings' ability to alter their form at will is not magical. However, there exists no known physical or biological explanation for how the sylvari can freely shrink or grow as required to mimic their targets.
Their peculiar ability aside, there is one more aspect to the sylvari that sets them apart from other races. Each sylvari possesses a set of phosphorescent, color-changing subdermal strips along their flanks called their auili'inawohanui'i (auili'i for short). Auili'i change their patterns and colors according to an individual's emotional state. While the patterns that manifest may change from one individual to the next, the colors that are displayed are standard across the species.
All sylvari possess a degree of conscious control over their auili'i. Total control can be learned, as evidenced by the sylvari who work for the Dominion's intelligence apparatus for undercover work, but the amount of effort required to do so means that most sylvari do not bother to do so.
Interestingly, sylvari auili'i play a central role in their native language. The auili'i provide important emotional context and subtext to the words that they speak. While an outsider may conceivably learn the language and understand, to a degree, the meaning behind what is said, they would be missing a whole layer of meaning unless they also understood how to interpret the displays of sylvari auili'i.
It is believed that this is the reason the sylvari had no system of prosaic writing until the arrival of the Dominion and the introduction of Tretalleri. Before then, they had a written script, but one that was used strictly for record-keeping. Stories, teachings, and other such material that might have emotional context were preserved primarily in oral tradition.