The 3d printed gun they have now can fire, but not well. It's an exceptionally poor gun, though they may someday make it worth something. As it stands, though, you can paint any real gun funny colors and make a fake gun look real. I don't think that is the problem.
I was unclear, but I meant more the shape than the color. Having wild tyedye colors (can you do that?) and an odd shape might make someone not realize it's a gun at all. This is my point. A box shaped 'gun' won't show up as anything suspicious on an x-ray scan at the airport. An airplane shaped one could be hidden in with a bunch of children's toys. It could be given to a toddler to carry (sans bullet) into a courtroom with the bullet inside the shoe (that has a metal buckle). There's all sorts of new and devious ways to sneak them around without being detected, in ways that you can't do with current guns that aren't spring-and-nail toilet paper reels.
The point is that a lot of these basic gun recognition tropes we take for granted right now - the shape, the weight, the metal, the color... all of that is thrown out the window. The only part that remains the same is the bullet. If you can't even recognize the firing device reliably... well isn't that a bigger problem than paper trails?
You can certainly tie-dye a gun! And that... would be the wierdest thing I will have ever seen.
Hey, I used the future perfect tense!
You weren't really unclear, I think people have this image of "a gun" and that's sort of what they expect. In reality, guns come in a nearly limitless variety of shapes, sizes, and styles. The first bullpup assault rifle, the Steyr AUG, is probably more recognizable to most people from movies that used it as a laser gun prop. It looked funky. The FN P90 by Fabrique Nationale looks like someone took a router to a piece of 2x6 lumber and used it to make a black plastic toy. It used to be fairly common for walking sticks to actually be loaded, single-shot guns. Spies have been building guns into pens for decades, even. I can't remember who for the life of me, but I remember someone being assassinated in Britain by having a ricin pellet fired into his leg from an umbrella.
I think a problem you're touching on is that, up until fairly recently, these kinds of things were actually illegal to own and possess. Some states have statutes on the books to make them illegal, but the U.S. has no national ban on weapons that look like other things. Likewise, most toy guns that look even moderately real come with a giant, orange tip on the end that alerts people that the gun isn't real. In fact, it's getting harder to get toy guns that look real. I remember having a few as a kid that looked somewhat genuine. I think some toy companies were worried that they might be "encouraging violent behavior."
It didn't do me any harm, but kids these days....
Anyway, for right now, I don't think the shape or style of the gun is going to be too much of an issue because they've made guns already that break all the rules. What you might be getting at that would be a serious problem are guns made to look like things that are not, in any way, shape, or form, guns. At least you'd need to be a pretty competent gunsmith to build a working gun out of a tape dispenser. If you can model anything, in 3d, from plastic and you can get a working gun, you can just 3d print a gun that looks like a pen, or a coffee thermos, or a smartphone. All of those would have to be fairly thick to accommodate even a .22 LR, but 3d printing heralds the end of the artisan.
That might be more of a problem. If anyone can simply produce anything and they don't need to go to a specialist, like a gunsmith, those kinds of weapons might become an awful lot more common. And they might look very, very convincing.