The Great Red Spot is a persistent anticyclonic storm, 22° south of Jupiter's equator, but why is it reddish?
It is not known exactly what causes the Great Red Spot's reddish color.
Are there updated data?
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It's red, because it's a 'sunburn'. The clouds in the red spot reach to higher altitudes than the surrounding and are more exposed to Solar UV radiation, which in turn changes the structure of some of the organic molecules etc. This is at least the explanation suggested by recent data from NASA's Cassini mission, see this 5 day old press release.