>>7090773http://dalong.net/review/kotobukiya/rex/rex_p.htmPainting it will make it look a lot better and like that promo pic you posted but an unpainted build will still have passable color separation
I'd recommend you start with something cheaper and easier and learn how to paint on those, before tackling the REX, though
Also read the guides in this Gunpla General OP for the basics and tips on the hobby