In this post, I got a cool way to create semi-transparent color filled boxes to put some info (tcolorbox worked great). I'd like to do the same but in beamer but with info in \titlepage, ie., one box for the title, one for the subtitle, one for the author and one for the institute. In the MWE below I'd like to have a semi-transparent colored box for each part of the \titlepage:
\documentclass{beamer}
\usetheme{Warsaw}
\setbeamertemplate{footline}[page number]
\setbeamercovered{invisible}
\setbeamertemplate{navigation symbols}{}
\usecolortheme{dolphin}
\useoutertheme[footline=empty,subsection=false]{miniframes}
\usepackage{graphicx}
\title{War of Cold Milk}
\subtitle{The return of Maggie}
\author{James Bond, Bart Simpson, and Darth Vader}
\institute{Springfield Institute}
\date{}
\begin{document}
% Main slide
\begin{frame}[plain]
\titlepage
\end{frame}
\end{document}
Is that possible? if so, could you use my MWE to show how please? any ideas are welcomed, thanks in advance,