Weapon Laws (Title 18, Article 12, CRS)
Colorado does not have purchase permits, gun registration, or gun-owner licensing. There is no waiting period mandated for firearm purchases, but private gun sales are illegal. Concealed handgun permits are shall-issue and loaded open carry is allowed (with some exceptions).
Colorado is a universal background check state. Magazines over 15 rounds capacity ('high capacity') are regulated. Denver has regulations which are more strict than the rest of the state, including banning 'assault weapons'.
Public buildings (open carry)
Local governments may prohibit open carry in public buildings or public areas, but must post 'no guns' signs at the entrance to public buildings or within the "specific area" where openly carried firearms are banned. 29-11.7-104
Public buildings (concealed carry)
You may not carry a concealed firearm into a public building at which security personnel and electronic weapons screening devices are permanently in place at each entrance. This does not apply to openly carried firearms and does not apply to private property with metal detectors. This does apply in Denver. 18-12-214
Some courts have taken it upon themselves to ban firearms in courthouses, regardless of other state laws. 13-10-112, see also ex. Colo. 1st Jud. Dist. LR 8. Prisons can ban firearms (17-20-124) and it is illegal to take contraband, including weapons, into a jail or prison. 18-8-203
You may not carry on the property or the offices of the general assembly or in which a legislative hearing or meeting or in which the official office of any member, officer, or employee of the general assembly is located. 18-12-105(c)
This also applies to concealed handgun permittees per 18-12-214. Capitol security webpage
No guns signs (private property)
'No guns' signs do not have the force of law. If you carry past a 'no guns' sign on private property, you may be asked to leave or disarm. Failure to comply would be trespassing. Signs on private property may cite 18-4-504 (trespassing). Local governments must post 'no guns' signs to ban open carry in public buildings. 29-11.7-104
You may carry a firearm in a bar or restaurant that serves alcohol. It is illegal to possess a firearm while under the influence of alcohol or a controlled substance, but there is no further definition. 18-12-106(d)
You may not hunt from a snowmobile nor have a firearm in your possession on a snowmobile unless the firearm is unloaded and in a case or carried in a scabbard. 33-14-117
It is illegal to carry a firearm on school property except that a concealed handgun permittee may have a handgun in a vehicle and, if the permittee is not in the vehicle, the handgun is in a compartment within the vehicle and the vehicle is locked (parking lot carry). 18-12-105.5 and 18-12-214
Campus carry (colleges/universities)
Firearms are not permitted at any public or private college, university, or seminary except when unloaded in a motor vehicle, in a traveler's private vehicle for lawful protection, or the handgun is concealed and carried by a concealed handgun permittee. Open carry is not permitted. 18-12-105.5
Loaded handguns are legal in vehicles without a permit, including when concealed in the vehicle on on the person. Denver can prohibit open carry in vehicles. Long guns cannot have a round in the chamber. 33-6-125
Only concealed carry with a permit is allowed in the non-sterile areas of an airport (outside the TSA checkpoints). Open carry is not permitted. 18-9-118, see also 18-9-115(4)
Only concealed carry with a permit is permitted on public transportation (buses, trains, etc.) 18-9-118, see also 18-9-115(4)
It is legal to carry concealed without a permit in your own dwelling, business, or on your property. This also applies to your own private automobile for self-defense while traveling. 18-12-105(2)(a)(b)
Marijuana is a Schedule I Controlled Substance under federal law. The ATF has found that users of marijuana are prohibited persons (see below) and may not possess firearms. Question 11.e. on the ATF Form 4473 used for firearm purchases specifically asks if buyers are marijuana users.