\documentclass{mnras}

\usepackage{graphicx}

\begin{document}

\begin{figure}
  \centering
  \includegraphics[scale=0.3]{example-image}
  \caption[]{text1: \\
    -- blue: 0.1 -- 0.36 \\
    -- green: 0.36 -- 0.98 \\
    -- red: 0.98 -- 3.5 \\
    -- purple: 3.5 + \\
    text2}
\end{figure}

\end{document}