This is a real story about a lot of people dying.
Lake Nyos is in a volcanically active area of Cameroon. It sits over a pool of magma, which leaks carbon dioxide gas into the lake. The gas dissolves in the water, but the lake becomes supersaturated. That means that any sudden disturbance of the water (caused by a small earthquake or landslide, for example, which is not unlikely in a volcanic region), can cause it to suddenly release a lot of gas.
A massive cloud of carbon dioxide pouring down a hillside can kill a lot of people.
On August 21, 1986, a limnic eruption occurred at Lake Nyos which triggered the sudden release of about 100,000 - 300,000 tons (some other sources state as much as 1.6 million tons) of CO2; this cloud rose at nearly 100 kilometres per hour (62 mph). The gas spilled over the northern lip of the lake into a valley running roughly east-west from Cha to Subum, and then rushed down two valleys branching off it to the north, displacing all the air and suffocating some 1,700 people within 25 kilometres (16 mi) of the lake, mostly rural villagers, as well as 3,500 livestock. The worst affected villages were Cha, Nyos, and Subum. Scientists concluded from evidence that a 100 m (330 ft) fountain of water and foam formed at the surface of the lake. The huge amount of water rising suddenly caused much turbulence in the water, spawning a wave of at least 25 metres (82 ft) that would scour the shore of one side.
Carbon dioxide, being about 1.5 times as dense as air, caused the cloud to “hug” the ground and descend down the valleys, where various villages were located. The mass was about 50 metres (160 ft) thick and it travelled downward at a rate of 20–50 kilometres per hour (12–31 mph). For roughly 23 kilometres (14 mi) the cloud remained condensed and dangerous, suffocating many of the people sleeping in Nyos, Kam, Cha, and Subum. About 4,000 inhabitants fled the area, and many of these developed respiratory problems, lesions, and paralysis as a result of the gases.
If you want coloured gas, you’ll need to use something other than carbon dioxide. However, do check whether the gas is water-soluble. If it is, a lake over a vent can cause a natural way for a lot of gas to gradually build up over a long time and then be suddenly, unpredictably released.