For the IJN it depends on the ship and time frame. They used a dark gray, but each yard had a slightly different tone. To determine the color, look through the "Tabular Record of Movement" for the ship on CombinedFleet.com.
Zuikaku's TROM shows her starting out assigned the the Kure Navy yard in 1941, a three-day stay at Yokosuka Yard in February (most likely not repainted) 1942, Drydocked in August 1942 at Kure (probably re-painted in Kure Gray), Stops at Yokosuka for three days to have a radar installed (probably not re-painted in Yokosuka gray, but possible?) at the end of Decemter 1942, drydocked again at Kura in June of 1943.... you get the picture. Shokaku's TROM is here, and the yards that had paint for you to keep an eye out for are Kure, Maizuru, Sasebo, and Yokosuka. White Ensign Models makes paints that are the exact match if you don't want to mix, or you can buy a set of paint chips from ShipCamouflage.com.