The more realistic answer is just that the artist didn't consider that and just drew their skin tones however the most in universe plausible answer is either an ecotone (the transition area between 2 biomes) they could be near a plains and due to their constant adventuring as a desert tribe they crossed over a lot reducing the sunlight they were exposed to so their skin tones didn't change as much (this is the most likely reason as too why its so varied between the tribe some of them would have ancestors that resided in the desert most of the time while others would adventure) or the other reason is rapid change from 1 biome to another due to some magical weather patterns and they could have had long seasons of rain, clouds etc which reduced their exposure to the sun's rays meaning they didn't need to adapt as much this one makes less sense tho as it doesn't explain the big variations between the tribesmen
Honestly, this is kinda a thing in a lot of manwha/manhua/manga no? I dont want to be that guy and just chalk everything up to racism or beauty standards in asian. But i also dont wanna think to deep over it either because theres no point, expecially in a site where our comment wont reach the creator lol and lets say if the creator do see our comments i can only assume majority are not complaining about it and those who do the numbers dont matter enough for the artist to make any changes...its one of those thing where "if it aint broke dont fix it ". And i dont mean to be the devils advocate here BUT it could have been worst. I rem reading a series that i cant rem the tittle to where the artist draw african characters very stereotypical and their language bubble had "oogaa boogaa" in it...yeah sorry for this nothing burger of a reply, i just wanted to chime in :D
Actually question why are some of the people who were BORN AND RAISED in the desert white like genuinely. Of course some of them have color in the skin but the ones that are shown more frequently are so light you would never guessed there from the desert.