R Markdown Colored Background - latex

Question

I have reproduced a simple example of an R Markdown presentation. Basically, I want to achieve a colored page that overlays both the headline and the footline. At the moment, only the headline is overlayed. The approach with a preamble in a tex file is needed in my full example. "Red.jpg" is simply a red image. There may be a better approach to achieve this. The reproducible code is as follows:
---
title: "Untitled"
author: "Author"
date: "21 6 2021"
output:
beamer_presentation:
includes:
in_header: preamble.tex
---
---
\begin{tikzpicture}[remember picture,overlay]
\node at (current page.center) {\includegraphics[width=\paperwidth]{red.jpg}};
\end{tikzpicture}
The code for the preamble is as follows:
\usepackage{tikz}
\setbeamertemplate{headline}{My Title}
\setbeamertemplate{footline}{Author \hfill \insertframenumber}
This really represents a simple MWE, without borthering about spacing of footline etc.
The current result is as follows:

Answer 1

You can use a plain frame without head- or footline:
---
title: "Untitled"
author: "Author"
date: "21 6 2021"
output:
beamer_presentation
header-includes:
- \usepackage{tikz}
- \setbeamertemplate{headline}{My Title}
- \setbeamertemplate{footline}{Author \hfill \insertframenumber}
---
---
# {.plain}
\begin{tikzpicture}[remember picture,overlay]
\fill[red] (current page.south west) rectangle (current page.north east);
\end{tikzpicture}