It's just marketing. I'll bet they either got a shipment of the wrong color plastic for some of their ballistic tips and made the best of it rather than sell it as blemished, or even ordered some just for this.
If they're trying to be authentic, I'll bet the rifle calibers are whatever they use for varmint rounds. Made basically to explode small critters after only a shallow penetration. That would work exactly the same on brains!
Yeah, the zombie stuff is goofy and kitschy, but that's part of the charm. No one sees zombie flicks because they're good. Heck, they're often not even Gary Busey levels of cinema. But they're still amusing, because you don't expect Shakespeare there.
Yeah, I'll buy a box. Hell, I'll buy a few. I do, after all, have a long term emergency box project. Mossberg, pistol (I wish the Z-Max came in 7.62 Tokarev! I may have to settle for 9mm and a Glock), and gas mask inside. Crowbar outside.
"In case of zombies, break glass. Retain prybar."