get rid of xcolor warnings

--- a/src/Doc/ProgProve/document/prelude.tex	Tue Mar 19 15:59:58 2013 +0100
+++ b/src/Doc/ProgProve/document/prelude.tex	Tue Mar 19 21:35:15 2013 +0100
@@ -25,7 +25,7 @@
 \renewcommand*\descriptionlabel[1]{\hspace\labelsep \textbf{#1}\hfil}
 
 % this should be the last package used
-\usepackage{color}
+\usepackage{xcolor}
 \definecolor{linkcolor}{rgb}{0,0,0.4}
 \usepackage[colorlinks=true,linkcolor=linkcolor,citecolor=linkcolor,
             filecolor=linkcolor,pagecolor=linkcolor,