This method uses the following GW colors:
Starting with a grey-primed model, I painted a coat of Mordain Blue (this may take several thin coats). Next, a layer of Ultramarines Blue was painted on, making sure to leave the darker color showing in the recesses. Ultramarines Blue was mixed 50/50 with Skull White and used to highlight the very edges of the armor. A wash of Asurmen Blue was applied over the armor to tie all the colors together and to deepen the recesses.
Here's a picture of the finished model, from the Heralds of Ultramar chapter. You can also use this formula to paint Ultramarines, who share the same armor color as the Heralds of Ultramar.