Solar UV variability is extremely relevant for the stratospheric ozone. It has an impact on Earth’s atmospheric structure and dynamics through radiative heating and ozone photochemistry. Our goal is to study the slope of the solar UV spectrum in two UV bands important to the stratospheric ozone production. In order to investigate the solar spectral variability, we use data from SOLSTICE (the Solar Stellar Irradiance Comparison Experiment) on board the Solar Radiation and Climate Experiment (SORCE) satellite. Datasets used are far UV (115–180 nm) and middle UV (180–310 nm), as well as the Mg II index (the Bremen composite). We introduce the SOLSTICE [FUV-MUV] colour to study the solar spectral characteristics, as well as to analyse the colour versus Mg II index. To isolate the 11-year scale variation, we used Empirical Mode Decomposition (EMD) on the datasets. The [FUV-MUV] colour strongly correlates with the Mg II index. The [FUV-MUV] colour shows a time-dependent behaviour when plotted versus the Mg II index. To explain this dependence we hypothesize an efficiency reduction of SOLSTICE FUV irradiance using an exponential ageing law.