Complex sentence: As plots were called in, the plotters used markers with the colour pointed to by the clock, which produced multi-coloured paths of markers on the table making easily followed "tracks"; the freshness of the information could be determined the colour of the latest marker.
Split into simpler sentences: As plots were called in, the plotters would use markers with the color currently being pointed to by the clock. Over time, this produced multi-color paths of markers on the table that produced easily followed "tracks", and the freshness of the information could be determined by looking at the color of the latest marker.