\addtokomafont{sectioning}{\color{sectioncolour}}
\titleformat{\section}[block]
{\usekomafont{sectioning}\usekomafont{section}
\tikz[overlay] \shade[left color=sectionfillcolour!40,right color=sectionfillcolour!5] (0,-0.28em) rectangle (\textwidth,1em);}
{\hskip 0.5em\thesection}
{1em}
{}
\titleformat{\subsection}[block]
{\usekomafont{sectioning}\usekomafont{subsection}
\tikz[overlay] \shade[left color=sectionfillcolour!40,right color=sectionfillcolour!5] (0,-0.28em) rectangle (\textwidth,1em);}
{\hskip 0.5em\thesubsection}
{1em}
{}
\titleformat{\subsubsection}[block]
{\usekomafont{sectioning}\usekomafont{subsubsection}
\tikz[overlay] \shade[left color=sectionfillcolour!40,right color=sectionfillcolour!5] (0,-0.28em) rectangle (\textwidth,1em);}
{\hskip 0.5em\thesubsubsection}
{1em}
{}