The army book explicitly says so. Look on the Alaitoc page. Of all the major craftworlds they are the most aggressive towards the Imperium.

u mean the codex?

Yh

Alrighty, let me get my fire retardent suit on...

okay, In order to determine what Craftworld we have to assess the target audience. They know they're gonna have the 40k fans hook-line-and-sinker. They want the bored WoW players. So, if Eldar are filling the "Elf" niche.. since they are Space Elves, then it goes to presume that they're gonna choose the most "Elfy" Craftworld with "Elfy" colors...

Stel-Uit: yeah it's green but the white helmets will clearly distiguish it from orks
Nacretinei: White could make a good contrast to the predominately black templars and the White/Blue is kinda High-Elven in look.
Tir-Val: It's got blue/yellow/green all the good makes of Elfy
lybraesil: Blue/white.. nuther high elf type
lyanden: If they want the Gold elf look
Alaitoc: Nuther possible high elf candidate

personally I'd go with Stel-Uit because they're a minor Craftworld. If you screw with their lore you wont be pissing a bunch of people off. You could just make the green darker, make the grey bright borderline silver and voila its a good contrast and support Elfyness.