Changing the color of itemize item in beamer

How about

\setbeamertemplate{itemize item}{\color{yellow}$\blacksquare$}
\setbeamertemplate{itemize subitem}{\color{orange}$\blacktriangleright$}

MWE

\documentclass {beamer} 

\mode<presentation>
{
    \usetheme{Madrid}
    \usefonttheme{professionalfonts} 
    \setbeamertemplate{itemize item}{\color{yellow}$\blacksquare$}
    \setbeamertemplate{itemize subitem}{\color{orange}$\blacktriangleright$}
}
\usepackage{hyperref}
\begin{document}
\begin{frame}
\begin{itemize}
    \item hey
\begin{itemize}
    \item hey
\end{itemize}
\end{itemize}
\end{frame}
\end {document}

The colors for the itemize symbols can be set as any other color in beamer by using \setbeamercolor, the symbolbols themselves by \setbeamertemplate.

The following code was used to get the result above.

\documentclass{beamer}

\setbeamercolor{itemize item}{fg=red}
\setbeamercolor{itemize subitem}{fg=blue}
\setbeamercolor{itemize subsubitem}{fg=cyan}

\setbeamertemplate{itemize item}[square]
\setbeamertemplate{itemize subitem}[circle]
\setbeamertemplate{itemize subsubitem}[triangle]

\begin{document}
\begin{frame}{itemize colors and symbols}
\begin{itemize}
    \item a red square
    \begin{itemize}
        \item a blue circle
        \begin{itemize}
            \item a cyan triangle
        \end{itemize}
    \end{itemize}
\end{itemize}
\end{frame}
\end{document}

This worked for me, at least in beamerposter: \setbeamercolor{itemize item}{fg=yellow,bg=white}.