I would prime in an acrylic paint in a grey colour. I usually prime with Vallejo Air Colour Gray Primer airbrushed on for a thin coat. You can also use a black or a white depending on how dark or light you want the subsequent colours to look (can just use spray cans but try get acrylic and not emulsion) For a really dark gritty model use black. For a model with lots of bright reds and yellows that won't be easy to paint over black use a white basecoat. Grey works out as a compromise - you can paint over it without any trouble but the colours don't look too super bright. This is how I'd do it if using acrylics which in my opinion gives you the best chance of creating a beautiful model. Try and get some Games Workshop washes to paint into the recesses of the model and help give it that sense of depth. They are all available online. For more painting tips for this kind of thing it's invaluable to visit the Games Workshop website as they have all sorts of guides. Probably best to practice on a few other models before jumping straight onto your Narin kit.