Here's a pile of interior bits for the Stormraven and two Rhinos.
Here they've been primed with Vallejo Surface Primer - Light Grey, then preshaded using Vallejo Surface Primer - Black. Over this I did a lot of light coats of Minitaire - Light Grey. These are thin coats so that the preshading shows through, to create the feel of depth on the panel lines and the corners and other details of the model. I added several layers of this colour, then finally did a very light layer of Minitaire - White to give it a little pop. I went with the light blue grey interior as it will create a nice contrast to the warmer golds and reds of the main army, both in colour and brightness. Plus, being brighter will help the interior details actually be more visible inside the unlit vehicles.
You can see here how bright the interior will be with the hatches open, to reveal the detail. If this were a darker colour, it wouldn't pop quite as much, and some of the detail wouldn't be as easily seen.