Could someone tell me how you get paint or skins for your guns?
or which guns can do this?

there's no paint mods for the weapons. you can only either buy limited edition skinned weapons from the shop in game with AP where these weapons are the same as normal weapons, just that they have a skin.

other that that, there's capsule shop where you can try to win a weapon (if you're lucky). there you can either go for euro (cheaper if you have battle points) or AP weapon.

Thanks!

Take into fact that some of these "skinned" weapons your wanting to know more about are also just not all skinned, for example the euro capsule/ap capsule varients have skins BUT they also have some if not alot of extra buffs to them example sidewinder vs an94 and g36 alligator vs g36, if you test both of these normal guns out then try their capsule varients there are remarkably huge differences.

But then there are skinned guns like the FAL cannon (not released yet) which is just a skinned para, and bg crusher which is just a Spas 15, along with the christmas style noel weapons and some others im sure. These give no additional stats, and are just meant to look cool, but honestly the placebo effect really plays into these guns.