I disagree with the prior writers: white necrons can work. You do not have to start out with gray. You just have to really make the shading a definition work out. Here, the paint is too thick, the lines on joints and in recesses are sloppy, and the remaining parts do not have anything done to get the pieces defined against one another. Even with a painted and textured base, it looks unfinished.
I agree with Sherlocko, if you start with a darker colour and then highlight you'll get a much better effect, and just remember that white is actually one of the hardest colours to work with.
I dont really like white necrons, dosen´t look "real". If u want i white, try makin it gry from the start, and then hi-lite it whit white, but a little bit thinner