It seems I am missing something while defining my title page in `beamer`. This was my first attempt (of course more sophisticated visually):

    \documentclass[aspectratio=169]{beamer}
    \usepackage{tikz}
    
    \mode<presentation>
    \defbeamertemplate*{title page}{mydefault}{%
    \begin{tikzpicture}
    \useasboundingbox (0,0) rectangle (\the\paperwidth,\the\paperheight);
    \fill[fill=red] (0,0) rectangle (\the\paperwidth,\the\paperheight);
    \node[anchor=north west,inner sep=0pt,outer sep=0pt] at (0,\the\paperheight) {This text should be in the upper left corner};
    \end{tikzpicture}
    }
    \mode<all>
    
    \begin{document}
    \begin{frame}
    \titlepage
    \end{frame}
    \end{document}

[![enter image description here][1]][1]

The issue was, that my red box with the text is not aligned to the upper left corner. If I put the same code into `\defbeamertemplate*{background}{mydefault}` the positioning is absolute to the upper left corner, but I need to have different backgrounds available for the user to select before he outputs a `\titlepage`. And just for reference, I want to avoid using `[remember picture,overlay]` and `current page`-nodes to avoid multiple compilations.


----------

I then realized, that maybe there is an alternate approach to reach my goal, which is to have 5 different backgrounds, 2 for titles, 1 for section title, one for regular pages and one for the presentation end page.

I read https://tex.stackexchange.com/questions/31316/different-backgrounds-for-title-and-normal-frames-in-beamer before, but it has the exact same issue with a non-absolute position. Then I came across https://tex.stackexchange.com/questions/459174/setbackgroundtemplate-on-title-page-within-custom-theme-file and https://tex.stackexchange.com/questions/317938/theme-with-a-different-footline-for-the-titlepage/323638#323638 so I continued to implement:

    \documentclass[aspectratio=169]{beamer}
    \usepackage{tikz}
    
    \makeatletter
    \def\ps@navigation@titlepage{%
    	\setbeamertemplate{background}[color][green]
    	\@nameuse{ps@navigation}}
    \addtobeamertemplate{title page}{\thispagestyle{navigation@titlepage}}{}
    \makeatother
    
    \mode<presentation>
    \defbeamertemplate*{background}{color}[1][red]{%
    \begin{tikzpicture}
    \useasboundingbox (0,0) rectangle (\the\paperwidth,\the\paperheight);
    \fill[fill=#1] (0,0) rectangle (\the\paperwidth,\the\paperheight);
    \node[anchor=north west,inner sep=0pt,outer sep=0pt] at (0,\the\paperheight) {This text should be in the upper left corner};
    \end{tikzpicture}
    }
    \mode<all>
    
    \setbeamertemplate{background}[color][red]
    
    \begin{document}
    
    \title{This shall be a green title}
    \begin{frame}
    \titlepage
    \end{frame}
    
    \begin{frame}
    This shall be a regular red content page
    \end{frame}
    
    \title{This shall be a blue title}
    \begin{frame}
    \setbeamertemplate{background}[color][blue]
    \titlepage
    \end{frame}
    
    \begin{frame}
    This shall be a regular red content page
    \end{frame}
    \end{document}

My problem is now that I don't get how to set another background for another title page, as my understanding is I can only pass one pagestyle to the title page with this way, any ideas? And would it be possible to do something like `\begin{frame}[red/blue/green]\titlepage\end{frame}` to pass the information into my template?

  [1]: https://i.stack.imgur.com/so2jm.png