Thinnner lines/areas of grey and maybe more white added to it to thin it down a little. That said I kinda like it like it is.
What scale is it, this can have an affect on how much shading yoiu should add.
The idea is to slightly darken the area around seem lines or edges. You can do the process in reverse. Paint the whole area in the darker shade and then center out the middle of each pannel with the correct colour. There were some Japanese modle making videos posted on a torrent site a while ago showing you how to do it among other things. A link was posted on M.W.
I have the files they are arround 100+ megs each. I have though of posting them to my web space during a weekend if the interest was there.