\documentclass{article}

\usepackage{biblatex,xcolor}

\addbibresource{biblatex-examples.bib}

\newcommand{\formatentry}{}
\patchcmd{\mkbibbrackets}% <cmd>
  {\begingroup}% <search>
  {\begingroup\formatentry}% <replace>
  {}{}% <success><failure>

\pretocmd{\printbibliography}% <cmd>
  {\renewcommand{\formatentry}{\color{red}}}% <precode>
  {}{}% <success><failure>

\begin{document}

\nocite{*}\cite{westfahl:space}

\printbibliography

\end{document}