(Spoiler) Why Lyse have so bright skin ?

[VargasVermillion]

There are light skinned highlanders, simple as that really. As for gaius, garleans used to be divided into various tribes until they united under a republic, simply put we don't know enough about them to have a clear cut input on genetic traits. Another example is how most garleans we have seen are quite large in build yet on the prima vista we see several midlander sized garleans as well as the female npc in gaius group (forgot her name).

[Vyrerus]

White people, some anyway, have this amazing thing that happens to them when they spend a long enough time in the sun... they tan. The longer you spend tanning, and the more you tan, the longer your skin will stay that way, until it finally goes past a certain point where it's considered, "sun damaged." When you spend enough of your life this way, as you age your skin stays tan and even darkens. Raubahn and Ilberd and other Ala Mhigans like them are older, close to their 50s, and have lived in the desert for most of their lives taking in that hot sun for days on end. Also some Ala Mhigans are just black or some sort of ethnicity/skin color that doesn't really come into play in the way it does in our world.

Lyse is young. She is in her mid 20s. She left the deserts of Ala Mhigo sometime after the Garlean occupation, as a young child, age 5. Grew up in Sharlayan. Then stayed in Eorzea in the Black Shroud, a forest that in some places blots out the sun with its canopy. She wears clothes that cover most of her body, particularly her face. She likely has rarely had a sunburn, let alone received lifelong high intensity sun exposure. Other Ala Mhigans like her include: Minfilia, Fordola, Widargelt, Erik the Alchemist from MNK quests, and a few others. SE plays pretty fast and loose with skin color, so I wouldn't put too much importance on it making sense. We as players get to make way less sense in character creation, making super porcelain seeker of the sun miqotes for instance.

As for Gaius, his darker hues likely arise from him spending a significant amount of time traversing The Burn, as well as being a dirty hobo for so long. Though there's also the idea that he's living on borrowed time at the power of that Elezen sorcerer who works along side him. This may have affected his pigmentation. Course this grouping goes to show that they don't put much thought into tans or skin color, since the lancer girl with him is paler than Lyse, despite traveling in the same circles. /shrug

[TheMightyMollusk]

By the Twelve, you can't just ask people why they're white.