On using beamer's colors in a Tikz picture

Question

For once, my question is short and simple: How do I use a beamer defined color in a tikz picture.

For instance, as beamer's documentation states, using beamer's colors is done as follows:

This text is {\usebeamercolor[fg]{alerted text} alerted}.
The following box uses the fore- and background of frametitles:
{
   \usebeamercolor[fg]{frametitle}
   \colorbox{bg}{Frame Title}
}

I would like to use the background color of the titleframe to fill an arrow shape as follows:

\usebeamercolor{frametitle}
\tikz
  \node[fill=\color{bg},single arrow] at (0,0) { };

Anyone has a clue? I'm being scold by xcolors:

! Argument of \XC@col@rlet has an extra }.

Answer 1

TikZ can use the color names provided by \usebeamercolor directly, you don't need to pass them to \color{...} first. Simply use color=fg for the foreground color, color=bg for the background, and so on:

\documentclass{beamer}
\usepackage{tikz}
\begin{document}
\begin{frame}
Hello

\begin{tikzpicture}
\frametitle{A Frame}
\usebeamercolor{frametitle}
\tikz
  \fill [fill=fg] (0,0) circle [radius=2cm];
\end{tikzpicture}
\end{frame}
\end{document}