\documentclass{article}
\usepackage{mathtools}
\usepackage{eulervm}
\renewcommand{\vcentcolon}{\mathrel{\mathpalette\eulervcentcolon\relax}}
\newcommand{\eulervcentcolon}[2]{%
\vcenter{%
\sbox0{$#1:$}\dimen0=.94781\ht0 \ht0=\dimen0 \box0
}%
}
\begin{document}
$\coloneqq$ $\vcentcolon\joinrel=$
\end{document}