While estimates made more than 5 deg. An advanced observer might wish to make transits systematically through an apparition, accompanying them with sketches, as has been done by the Jupiter Section in the past. Systematic verbal estimates of colours of belts and zones may be useful, if made with a reflecting telescope of at least 250 mm. There are subtle long-term changes and sometimes dramatic changes, as when a zone adopts a vivid yellow colour. Although there is considerable scope for artefacts, e.g. due to subjective contrast effects, this is also true of colour CCD images, e.g. due to use of different filters and image-processing.
Colours are easier to discern in twilight.
They are inevitably subjective so are only worthwhile if done systematically and repeatedly within the context of a more detailed observational programme. Observers who make colour or intensity estimates should tabulate and average them for the whole apparition, and note any significant changes. The transits, shadow transits, eclipses, and occultations of the four galilean satellites are among the most striking phenomena that a beginner can observe. There are few other systems which demonstrate planetary motions so directly to the eye. Io a faint grey, and Europa usually invisible against the bright clouds.