Japanese warships were painted in various colors depending upon their home port, or last port where they may have received a major refit.
Sasebo, Kure, and Yokosuka and Maizuru Naval Yards, the principal ones during the war, all used different shades of gray, and later in the war, there were 2 shades of greens used on aircraft carriers, in addition to a tan and various grays used for flight deck camouflage.
I have a couple of recommendations, first, tell us what ships you're working on, and pick up the Snyder and Short IJN Paint Chip set. It's a must have.
http://www.shipcamouflage.com/ijn.htm
Regards,
Jeff Herne
Modelwarships.com