Toxic

A green cloud takes the figure and leaves a silhouette. The subject is present and mostly hidden, which is a different kind of threat from a subject you can see clearly.

Green at this saturation is a warning colour. Nothing in ordinary weather is that colour, so the air itself reads as the danger rather than as atmosphere.

The vapour is lit from inside. A source within the cloud makes it glow as a volume, so the green comes from the air rather than from a gel on a lamp pointed at it.

Density matters more than colour. Thick enough to reduce the figure to an outline, thin enough that the outline still reads, which is a narrow band worth naming in the prompt.

Movement sells it. Vapour rolls and folds rather than drifting evenly, and the parts nearest the internal source stay brightest as it moves.

Use it for a reveal, a threat, or a figure emerging from something. It suits a slow approach where the silhouette resolves over a few seconds.

Skip it when the face has to be read. The cloud exists to take the face away, and a version thin enough to show one has lost the point.

Fog cuts the distance and flattens everything behind the subject, erasing the background while leaving the subject clear. It is a colourless condition of the whole scene.

The check is what is hidden and what colour it is. Green vapour swallowing the subject means Toxic. Neutral density erasing the background while the subject stays sharp means Fog.

The model lights the cloud from outside, and it reads as green smoke on a set, which is the trap the entry warns about. Say the acid green light comes from within the cloud.

The other failure is clear air with a green grade over it. Ban clear air and natural colour, and say the figure is reduced to a silhouette inside the vapour.