Modifié Voici une solution, élaborée à partir de https://tex.stackexchange.com/a/167702 celle [postée sur un autre site](https://tex.stackexchange.com/a/167702) :
\documentclass{article}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{color}
\newcommand\twocolour[2]{\hspace*{0pt}\rlap{\textcolor{red}{#1}}#2}
\DeclareUnicodeCharacter{00E9}{\twocolour{\'{e}}{e}}
\DeclareUnicodeCharacter{00C9}{\twocolour{\'{E}}{E}}
\DeclareUnicodeCharacter{00C1}{\twocolour{\'{A}}{A}}
\DeclareUnicodeCharacter{00E1}{\twocolour{\'{a}}{a}}
\DeclareUnicodeCharacter{00D3}{\twocolour{\'{O}}{O}}
\DeclareUnicodeCharacter{00F3}{\twocolour{\'{o}}{o}}
\DeclareUnicodeCharacter{00E8}{\twocolour{\`{e}}{e}}
\DeclareUnicodeCharacter{00C8}{\twocolour{\`{E}}{E}}
\DeclareUnicodeCharacter{00C0}{\twocolour{\`{A}}{A}}
\DeclareUnicodeCharacter{00E0}{\twocolour{\`{a}}{a}}
\DeclareUnicodeCharacter{00D2}{\twocolour{\`{O}}{O}}
\DeclareUnicodeCharacter{00F2}{\twocolour{\`{o}}{o}}
\DeclareUnicodeCharacter{00C7}{\twocolour{\c{C}}{C}}
\DeclareUnicodeCharacter{00E7}{\twocolour{\c{c}}{c}}
\DeclareUnicodeCharacter{00CA}{\twocolour{\^{E}}{E}}
\DeclareUnicodeCharacter{00EA}{\twocolour{\^{e}}{e}}
\DeclareUnicodeCharacter{00C2}{\twocolour{\^{A}}{A}}
\DeclareUnicodeCharacter{00E2}{\twocolour{\^{a}}{a}}
\DeclareUnicodeCharacter{00CE}{\twocolour{\^{I}}{I}}
\DeclareUnicodeCharacter{00EE}{\twocolour{\^{i}}{\i}}
\begin{document}
Les canards écrivent avec \LaTeX
è
È
É
Á
á
À
à
Ó
ó
Ò
ò
Ç
ç
Ê
ê
Â
â
Î
î
\end{document}
![alt text][1]
[1]: https://texnique.fr/osqa/upfiles/Screen_Shot_2019-12-03_at_10.34.11.png<img src="/upfiles/Screen_Shot_2019-12-03_at_10.34.11.png" style="width: 100%;"/>
