Union guns were to be painted a "liquid olive" color in the field. Before being painted they were often left coated with linseed oil thinned with turpentine so they would dry. Once they left storage they were to be painted. If a person wants to duplicate the color, you have to be careful. The liquid olive color is slightly lighter than the olive drab color now in use in the military. If you use a historic recipe, it turns out to be a mustard color. This is due to differences between historic and modern yellow ocre ingredients. Ed Bearss says just about any green or brown color would probably be acceptable. Confederate guns were often captured guns and were probably used as captured. As an aside, the Union army white washed their guns used in the grand review in 1865.