That's the point. They're adding customization for the current races in Shadowlands. It's customization for Night Elves. Highborne were just a caste of Night Elves.
That said, they actually did have some differences from the rest of the Kaldorei.
-They usually had lighter skin, golden eyes (many times silver, as well), and silver hair.
-Some dyed their hair silver to be more like Azshara - Vashj was implied to do this in the War of the Ancients trilogy, but it looked more like a dull gray. In
this image Vashj had dull silver hair with light indigo streaks in it, and in
this image she has blonde streaks in her hair.
-Dath'Remar Sunstrider had auburn hair with blonde streaks in it -
as seen here and
here (which looks quite different to his
High Elf form).
-Also in
this image you can see they have black hair, white hair, blonde hair, and even red hair.
-Hair colors like blue, green, purple, and teal are much less common among the Highborne.
-They could have a wide range of skin tones, from
blue to
lavender to all the ones we see in-game.
-That's also why the Nightborne have the hair colors they do, since they are entirely born from generations of Highborne. The Nightwell certainly changed their eye colors and skin tones, but there are similarities between some Nightborne and Highborne skin tones as well.
Since they're going to add more customization for all the races, and Highborne have been part of the alliance since Cataclysm, this is a perfect fit.