Well, for depth, it's pretty easy.
The 'sun' is upwards. So, color the lower parts in a darker color, and the higher parts brighter.
On 'flat'...
Well, if you just cannot stand flat color then since you're running Photoshop, you could always add a cloud map on the plating (but not on the edges or slots) on soft light, hard light, overlay, whatever fits your fancy, and lower the opacity until it looks okay to you.
Or, you could, I donno, use the pencil at 10% opacity and brighten/darken parts to your heart's content.
Really, if something is flat, I don't see why a flat color is a huge problem since the 'sun' is upwards.