A pure volcanic sulphur dioxide gas plume appears typically (light) green in the Ash RGB images.
Note that it might be yellow at high latitudes and for large viewing angles near the edge of the full Earth disk. The mixture of SO2 gas and ash plume may be also yellow.
The Ash RGB image below shows an eruption of the Mount Etna. One can recognize the hot lava in the crater (dark blue) and a plume released by the Mount Etna, which is magenta close to the crater and green further downstream (with some yellow patches in between). The green colour indicates pure SO2 gas plume, the magenta colour indicates volcanic ash plume; the yellow patches are likely to be a mixture of both.
In the image below one can see the eruption of Nyamuragira volcano in Africa at November 2006. It released large amounts of sulphur dioxide. The image shows the sulphur dioxide cloud in green colour. The brown and black colours correspond to clouds, the bluish colours to the surface.
Meteosat, SEVIRI Ash RGB for 29 November 2006 11:10 UTC (source: EUMETSAT, Image Library)
Note that the detection of SO2 is more effective if
• the SO2 concentration is higher,
• the SO2 cloud is purer (without ash particles)
• the temperature difference between the underlying surface and the SO2 cloud is higher.
Explanation of the colours of a volcanic sulphur dioxide gas plume (see also the recipe on the left side):
The key channel combination of sulphur dioxide gas plume detection is the (IR10.8-IR8.7) brightness temperature difference (visualised in the green colour beam).
• The red signal is medium or low in lack of thin ash cloud. (Weak in case of thin ice clouds.)
• For an SO2 gas plume the green component is high, (because the SO2 gas strongly absorbs radiation at around 8.7 micrometer, while it does not absorb the radiation at 10.8 micrometer).
• The blue signal depends on the temperature and the transmittance of the gas plume, and the temperature of the underlying surface or cloud layer.
As a consequence the pure sulphur dioxide gas plume appears (light) green. It appears yellow in case it is mixed with ash.