The colour index dependence on the Mg II index and time. Date is represented by the colour bar (shown on the right). Data points for November 2012, August, September, October and November 2013 and January 2014 are not shown, as SSI data is not available for these months. Note that the strong correlation between the [FUV-MUV] colour and the Mg II index shows two slightly different slopes, corresponding to the descending phase of solar cycle 23 and to the rising phase of solar cycle 24.