Hydrogen Cyanide is actually a colorless liquid that would evaporate in the sunlight. You may be looking for Prussian blue, which has got a lovely blue color, b

Hydrogen Cyanide is actually a colorless liquid that would evaporate in the sunlight. You may be looking for Prussian blue, which has got a lovely blue color, but isn't very toxic. Anyway, this is orthogonal to the effectivity of your analogy, but I thought you may want to know.