Why are acids usually depicted in fiction as green? In cartoons, etc., acid is generally shown as vividly green, as in this scene from the Simpsons:



We even have the colour "acid green".

But when I used to use acids in chemistry class, they weren't bright poisonous green. They were mostly colourless, as, for example, this picture of sulfuric acid from Wikipedia:



So where does this idea of green acid come from? Are there acids which are actually green? Or perhaps acids are green when highly concentrated?
The green is the color of bile which usually accompanies gastric acid.  Acid  in its original context referred to a bitter taste.  Bitter tasting compounds, such as vinegar, is what led to the modern chemical definition.  The chemical definition is less likely to have influenced the populace than the experience associated the somewhat bitter, acidic, taste of gastric acid accompanied by bile.