Beamer itemize with odd even variation

Question

i would like to obtain the following effect automatically

\documentclass{beamer}

\definecolor{color1}{RGB}{255,0,0}
\definecolor{color2}{RGB}{0,255,0}

\begin{document}
\begin{frame}
\begin{itemize}
\setbeamercolor{itemize item}{fg=color1}
\item
\setbeamercolor{itemize item}{fg=color2}
\item
\setbeamercolor{itemize item}{fg=color1}
\item
\setbeamercolor{itemize item}{fg=color2}
\item
\end{itemize}
\end{frame}
\end{document}

Answer 1

I think with this approach you can get it:

\documentclass{beamer}
\usepackage{etoolbox}

\definecolor{color1}{RGB}{255,0,0}
\definecolor{color2}{RGB}{0,255,0}

\newcounter{myitem}
\setcounter{myitem}{1}
\renewcommand<>{\item}[1]{\only#2{
\ifnumodd{\themyitem}{%true
\setbeamercolor{itemize item}{fg=color1}\stepcounter{myitem}
}{%false
\setbeamercolor{itemize item}{fg=color2}\stepcounter{myitem}
}
\beameroriginal{\item}{#1}}}

\begin{document}
\begin{frame}
\begin{itemize}
\item text
\item text
\item text
\item text
\end{itemize}
\end{frame}
\end{document}

Graphical result:

IMPROVEMENT

To activate or disable the alternate item coloring you can adopt a trick like this one:

\documentclass{beamer}
\usepackage{etoolbox}

\definecolor{color1}{RGB}{255,0,0}
\definecolor{color2}{RGB}{0,255,0}

\newif\ifitemcolor % <= create a new conditional
\itemcolortrue %<= set it to true to activate the coloring

\newcounter{myitem}
\setcounter{myitem}{1}
\renewcommand<>{\item}[1][]{\only#2{
\ifitemcolor%
\ifnumodd{\themyitem}{%true
\setbeamercolor{itemize item}{fg=color1}\stepcounter{myitem}
}{%false
\setbeamercolor{itemize item}{fg=color2}\stepcounter{myitem}
}\fi%
\beameroriginal{\item}{#1}}}

\begin{document}
\begin{frame}
\begin{columns}
\begin{column}{0.48\textwidth}
\itemcolorfalse %<= set it to false to disable the coloring
\begin{itemize}
\item
\item text
\item text
\item text
\end{itemize}
\end{column}
\begin{column}{0.48\textwidth}
\itemcolortrue %<= set it to true to re-activate the coloring
\begin{itemize}
\item 
\item text
\item text
\item text
\end{itemize}
\end{column}
\end{columns}
\end{frame}
\end{document}

In the example I created two columns with two lists: the list on the left has the alternate coloring disabled while the list on the right does not. Moreover the re-definition of the \item with additional [] allows to not insert text, as did for the two first elements in both the lists.

The graphical result is:

Answer 2

Yo can also have two definitions of the \item and altermnate back and forth between them. The alternating coloring is enabled via \AlternateColors called within the itemize environment. Without this, the default coloring is produced.

\documentclass{beamer}

\definecolor{color1}{RGB}{255,0,0}
\definecolor{color2}{RGB}{0,255,0}

\let\OldItem\item
\newcommand{\itemTwo}{%
    \setbeamercolor{itemize item}{fg=color2}\OldItem%
    \def\nextitem{\itemOne}%
}%
\newcommand{\itemOne}{%
    \setbeamercolor{itemize item}{fg=color1}\OldItem%
    \def\nextitem{\itemTwo}%
}%
\newcommand{\nextitem}{\itemOne}%
\newcommand*{\AlternateColors}{%
    \def\item{\nextitem}%
}%

\begin{document}
\begin{frame}
\begin{itemize}\AlternateColors% This enables the alternate coloring
  \item
  \item
  \item
  \item
\end{itemize}
\end{frame}
\end{document}