While describing a new language, [n] or [s] delimiter types are not recognized. Below, you find an example which reproduces the issue. 

example.tex:
\documentclass{article}
\usepackage{listings}

\usepackage[usenames]{color}
\usepackage[normalem]{ulem}

\definecolor{gray}{rgb}{0.4,0.4,0.4}
\definecolor{darkblue}{rgb}{0.0,0.0,0.6}
\definecolor{cyan}{rgb}{0.0,0.6,0.6}

\lstset{
  basicstyle=\ttfamily,
  columns=fullflexible,
  showstringspaces=false,
  commentstyle=\color{gray}\upshape
}

\lstdefinelanguage{XML}
{
  morestring=[b]",
  morestring=[s]{>}{<},
  morecomment=[s]{<?}{?>},
  stringstyle=\color{black}\textbf,
  identifierstyle=\color{darkblue},
  keywordstyle=\color{cyan},
  morekeywords={greeting}% list your attributes here
}

\begin{document}

\begin{lstlisting}[language=XML, float,
    basicstyle=\scriptsize,
    frame=lines,
    breaklines=true,
    tabsize=2]
<greeting>Hello</greeting>
\end{lstlisting}

\end{document}