Now, I know the Fallout and Doom pre-order packs come with "skins". Will guns be colored ONLY by skins? Or will there be coloring options?

I certainly HOPE you can color weapons, since it's all you see of your character in-game.

noone knows, i suppose there are several skins you can apply, not sure if you can colour them but it's possible

Good idea, but I think guns would be limited to a few skins only.
Information on this subject however hasn't been releases, so my statement is simple speculation.

Eh, really speculation can be thrown either way here, after all one of this game's core features is customization. So, it seems only rational that you could do a lot with your gun. But on the other hand, perhaps it's not needed, since your character will be so unique. I myself hope for the former, after all who doesn't love a sick looking, totally unique gun?

The resistance guns have graffiti on them that could easily be color customizable, and with the whole personalization theme of the game, it would seem right at home. Im not sure what you could have on the security side that is comparable, though. Perhaps the body of each gun could be painted a color of your choice. To get an idea of what I mean, if you look at the Hockler Machine Pistol, from the http://bethblog.com/index.php/2010/08/17/brink-pre-order-bonuses-announced/, it has a body that is blue-green.

I'm guessing they will just have skins. If it had coloring options they probably would have had that and some decals for the preorder bonuses. I would love to color my gun rainbow but I think it will just be skins.

I think I saw a colouring-option in the demo, but I wasn't really allowed to get into detailed customisation because it was really late and them guys wanted to finish the day already.

For Security it's most likely just the colour, white, black, red, blue, green and so forth. For the Resistance it's graffitis, as mentioned above. I think you can actually see some different skins in the various videos available. Iirc there is, for example, a green and an orange graffiti for the TDI Vector SMG lookalike.

So far we haven't seen any option for coloring it. I wouldn't mind seeing them add it in though. I mean with all the talk about the color and cartoon-ish look of Brink it be a little sad if you couldn't put them on your gun.

From the link you have given, I believe that you can't really customize your gun, and only skins are available. BTW, that soda can silencer is awesome!