In movies I often see green used to color dangerous and/or poisonous substances colored green, like green clouds of gas considered poisonous, and green acid considered dangerous. Can anyone explain this, or explain where this came from?
Because it was forever linked to poison by Radium and Green pigments in the 1800's. Paris Green and Scheeles Green in particular helped cement this association early on.
It was clear from the beginning large doses of Radium was dangerous, the Curies made it clear early on, but it was commonly held that in small doses Radium was beneficial.
Those who painted radium watches suffered from drastic Radium poisoning, and would glow green. The radium girls destroyed the reputation of Radium, giving us the classic radioactive green glow association
What Made Green Pigments So Toxic?
Both were arsenic compounds that produced popular shades of green, known for their brilliance. They were excessively used
Paris green (copper(II) acetate triarsenite or copper(II) acetoarsenite) is an inorganic compound. It is a highly toxic emerald-green crystalline powder3 that has been used as a rodenticide and insecticide, and also as a pigment, despite its toxicity.
So much so that Paris Green was later used as an insecticide. It was used to kill mosquitos, and in the Paris sewers to kill rats.
How Did They Respond in the 19th Century?
Green gained an ominous reputation, with tales of women in green dresses collapsing, newspaper printers being overcome by fumes, and children wasting away in bright green rooms
It's widely implied that these green pigments contributed to Napoleons death due to the lavish green suite he slept in.
There are also reports it caused health issues with famous artists who used the pigment, such as blinding Monet
Other Dangerous Colours
There are other colours with similar associations, I would recommend this Ted Ed video
This comes from Real Life; see Why is green the color of poison? for example.
Have you ever wondered why poison is so often associated with the color green? Take movies or video games, for example: even the vials containing poison would frequently be made of a green glass. Sure, there are poisonous plants and all, but look no further than the 18th century, when some toxic green pigments forever ruined the reputation of the color.
The beloved pigments of Cézanne and Monet, who excessively used them in paintings, Scheele’s and Paris green were so toxic that they were routinely used to kill the rats. So toxic, in fact, that Monet’s blindness, Cézanne’s diabetes, and even Napoleon’s death are still blamed on them.
quote from Wikipedia on Chlorine discovery
Scheele observed several of the properties of chlorine: the bleaching effect on litmus, the deadly effect on insects, the yellow-green color, and the smell similar to aqua regia.