My impression is that the background should be of a color(s) that has (have) as little opportunity of being confused with a/o sharing with colors of the subject. Thus colors in the firearm such as blues (action case color), reds (case color and wood), greys (action), and browns (wood) should not be used as background in photos of a firearm. I think that some of the photos with a dark green background work quite well. That being said lighting is prolly most important in that it shouldn't be so direct as to create reflection but light and diffused enough to show the details. Also the background color(s) should not detract attention from the subject (yellow should be dismissed but straw or mustard might work well).