The color code for HE loaded munitions changed in 1942. Prior to this, all munitions, land mines, grenades, bombs, 60 and 81mm mortar and artillery projectiles from 37mm up, were painted yellow with black lettering. The code then changed to Olive drab with yellow lettering for HE loaded munitions as the warhead on the rocket shows. There are exceptions for some Navy used ordnance. Their bombs were painted Gray with a colored dot between the lugs.
A number of munitions that were originally yellow were repainted over the yellow with OD. Some of the more commonly found, are the MK II pineapple grenades. If you watch the move "The Thin Red Line" about Marines in the Pacific early WWII, they are carrying yellow MK IIs.
Imagine how easy it would be to find an ammo dump on land or on an aircraft carrier deck from the air, if all the bombs were painted yellow.