thanks to this site I was able to get a functional implementation of scrlayer-scrpage.
\documentclass[12pt,headsepline,footsepline,plainfootsepline,plainheadsepline]{scrreprt}
\renewcommand{\familydefault}{\sfdefault}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{geometry}
\geometry{verbose,tmargin=3.5cm,bmargin=3.5cm,lmargin=3cm,rmargin=2.5cm,headheight=33pt}
\usepackage{graphicx}
\usepackage{xcolor,calc}
\definecolor{mygreen}{RGB}{23,156,125}
\usepackage{kantlipsum}
\usepackage{scrlayer-scrpage}
\setkomafont{pageheadfoot}{\upshape}
\setkomafont{pagehead}{\slshape}
\setkomafont{headsepline}{\color{mygreen}}
\setkomafont{footsepline}{\color{mygreen}}
\pagestyle{scrheadings}
\automark{chapter}
\ihead{\leftmark\hfill \includegraphics[height=1cm]{IMG/Logo.jpg}}
\ohead{}
\chead{}
\ofoot*{\thepage}
\cfoot*{}
\chead{}
\ihead*{\leftmark\hfill \includegraphics[height=1cm]{IMG/Logo.jpg}}
\begin{document}
\chapter*{Some chapter}
\tableofcontents{}
\part{Hello}
\chapter{Some chapter}
\kant[1-10]
\end{document}
This works as intended but I would like to have two changes. The chapter "Some Chapter" which does not show in the TOC should have its title in the header alongside with the logo but without the page number. And the "parts" page should have two logos in the header instead of title and logo. I tried it with
\newpairofpagestyles{speciallayout}{
\ihead{\leftmark\hfill \includegraphics[height=1cm]{IMG/Logo.jpg}
\ofoot{}
\ofoot*{}}
but this didn't work.
Thanks in advance
Jon