Is it possible to change the color of the text according to the background color?

Question

I'd like to automatically change the color of the text according to the background color.

I've used tikz to create the background (the shape is only an example) but any other way to do it is welcome.

\documentclass{beamer}
\usepackage{tikz}
\usetheme{Madrid}
\usebackgroundtemplate{%
    \begin{tikzpicture}
        \coordinate (A) at (0,0);
        \coordinate (B) at (0,\paperheight);
        \coordinate (C) at (\paperwidth,0);
        \fill[blue] (A) -- (C) -- (B) -- cycle;
    \end{tikzpicture}
}
\begin{document}
\begin{frame}
    \frametitle{Example}
    Could this text be automatically white on blue and blue on white?
\end{frame}
\end{document}

Answer 1

The idea is simple: use some nontrivial blending mode.

You might want to

• tune the colors, so that "blue" minus "yellow" is "white"
• modify the background, so that the lower right corner looks right.

I test the document with Preview and Adobe Reader on Mac.

---

\documentclass{beamer}
\usepackage{tikz}
\usetheme{Madrid}
\usebackgroundtemplate{
    \begin{tikzpicture}
        \coordinate (A) at (0,0);
        \coordinate (B) at (0,\paperheight);
        \coordinate (C) at (\paperwidth,0);;
        \coordinate (D) at (\paperwidth,\paperheight);
        \fill[blue] (A) -- (C) -- (B) -- cycle;
        \fill[white](D) -- (C) -- (B) -- cycle;
    \end{tikzpicture}
}
\begin{document}

\begin{frame}
    \frametitle{Example}
    \pgfsetblendmode{difference}
    \color{yellow}Could this text be automatically white on blue and blue on white?
\end{frame}

\end{document}

Edit:

As @Werner suggested, if you do not know how to fix the footline, add this line:

\addtobeamertemplate{footline}{\pgfsetblendmode{normal}}{}

Change footline to anything else if you find them unexpected.

On the other hand, if you think it is awesome,

\addtobeamertemplate{navigation symbols}{\pgfsetblendmode{difference}}{}