Real camera guys will yell at my terminology but...
It's because of automatic camera controls - white balance, metering, and exposure settings. Most point-and-shoot cameras look at the lightest spot in the view and call that "white" and the darkest spot and call it "black" - everything else is relative to those two shades.
Digital cameras normally skew colors to adjust for the type of lighting. Standard incandescent bulbs put out a dirty brown light; digital cameras adjust by skewing everything toward blue, which makes the color rendition fairly accurate. Fluorescent bulbs are bluish; photos shot under them are skewed toward yellow to balance.
In a fairly low-contrast scene, a camera can't get a proper automatic white balance, and skews the color based on what it *thinks* your lighting conditions are. So if you're shooting against a green background and the brightest spot on the scene isn't actually white (the brightest areas appear to be the reflections of the lights on the bottom of the revolver cylinder, and a line on the right side of each gun) it's going to shift everything a little bit to compensate. Neither the revolver cylinder nor the guns are actually white, but the camera pushes the brightest spot to be white, regardless of what it does to the rest of the image. There are also some spots in the green felt that show up white; the white balance might have skewed the whole image a little red, which would make the gun look brown.
If you want good color rendition in this scene, you'll need to manually set the white balance and exposure, or make sure there is something that is actually white in the scene.
Look at the photo of the stippled glock - the background has a lot of actual white in it: the white around the red cross, the white rocks... When the camera shifts these spots to "white", they are actually white, so the color is more accurate.
The Gen4 Glock photo is washed out a bit. Look for the white in that photo - there isn't much and what is painted as white in the photo isn't really white in real life. The brightest spot appears to be a reflection off the slide above the slide-stop lever. The slide isn't actually white, of course, so for the camera to paint that section white, it has to skew the rest of the photo, washing out the rest of the image.
On my point-and-shoot camera (Lumix DMC-ZS8), I have an option called "metering mode" - I can set it to "area" "center weighted" or "spot" - Setting it to "spot", if I pointed the camera at the bright spot on the slide of the Gen4, the photo would be much darker than if I pointed it at the trigger guard. "Spot" metering lets me focus on one spot and lock the automatic focus and exposure settings with a half-press of the shutter button, then shift to the scene I want to shoot and "fire". To shoot that Gen4 as accurately as possible without messing with the lighting, I'd lock the settings on the bright spot, then shift the view until it was centered as I wanted it. The quality wouldn't be perfect, but it would be an improvement over basic point-and-shoot with fully automatic settings.
Seawolf is right - it's a PITA to shoot photos of glossy objects without carefully controlling the ambient light.
Incidentally, photography (especially outdoor) and firearms handling have a heck of a lot in common, especially in handling. Trigger control, breath control, grip, stance, point of focus, etc. all carry over. The feedback of "trigger slap" on a digital camera is immediate - shooting photos with the automatic stabilization feature shut off made me much more aware of failures to isolate the trigger finger from the grip. When you can't go out and shoot at things with a gun, using a camera is a good alternative.
