Changing the colour of Beamer \titlepage using a .sty file

Good question! And to be honest, I do not know the answer, but I use a workaround in my own style files. In the .sty file, I define a wrapper macro for the titlepage similar to

      \newcommand{\insertoxfordtitle}{%
        \setbeamercolor*{background canvas}{fg=oxfordblue,bg=oxfordblue}
        \begin{frame}[plain]
        \titlepage
      \end{frame}
      \setbeamercolor{background canvas}{fg=oxfordblue,bg=white}
    }

In the .tex file I replace the title frame by \insertoxfordtitle.

Relevant changes to the .sty file:

    % Reformat Title Page to our own specifications
    \setbeamercolor{title}{fg=white}
    \setbeamercolor*{subtitle}{fg=white}
    \setbeamercolor*{date}{fg=white}

      \newcommand{\insertoxfordtitle}{%
        \setbeamercolor*{background canvas}{fg=oxfordblue,bg=oxfordblue}
        \begin{frame}[plain]
        \titlepage
      \end{frame}
      \setbeamercolor{background canvas}{fg=oxfordblue,bg=white}
    }

   \setbeamertemplate{title page}{
       \begin{columns}
          \column{0.7\textwidth}
          \usebeamerfont{title}\usebeamercolor[fg]{title}\inserttitle
          \ifx\insertsubtitle\@empty
          \else
             \vskip2ex
             \usebeamerfont{subtitle}\usebeamercolor[fg]{subtitle}\insertsubtitle
          \fi
          \ifx\insertauthor\@empty
          \else
             \vskip2ex
             \usebeamerfont{author}\usebeamercolor[fg]{author}\insertauthor
          \fi
          \ifx\insertdate\@empty
          \else
             \vskip2ex
             \usebeamerfont{date}\usebeamercolor[fg]{date}\insertdate%
          \fi
          \ifx\insertinstitute\@empty
          \else
             \vskip2ex
             \usebeamerfont{institute}\usebeamercolor[fg]{institut}\insertinstitute%
          \fi
          \vfill
          \column{0.3\textwidth}
          % \includegraphics[width=0.5\textwidth]{pic1}\\
          \vskip5pt
          % \includegraphics[width=0.5\textwidth]{pic2}\\
          \vskip5pt
          % \includegraphics[width=0.5\textwidth]{pic3}
       \end{columns}
    }