Actually, most of my Karists are purple... The white armoured force on the website was painted by a studio.
I did do this guy just for fun a while back:
His armour was painted with Vallejo Light Grey, and then highlighted with white, mixing in light grey to blend back down into the creases. The end result was a bit greyer than I originally intended, but that's basically just down to the amount of white area that you leave... using more white and less grey alters that balance towards a whiter finish. You could do the same with Army Painter Ice Storm instead of the grey for more of a bluey-white like the studio force.
For a faster option, Army Painter's Holy White speedpaint also seems quite good, although I've only used it on small sections so far, not for armour.
Or there's the option of just spraying the whole thing white and then lining in the creases with a light grey or blue. That gets a starker finish, but looks reasonable if it's done neatly.
Working up some tutorials for easy versions of the studio paint schemes is something that's been on my to-do list for a while... definitely need to find time to get it done, particularly as we will be showcasing more colour scheme options in the V2 rulebook.