pgfgantt xcolor Error undefined color

documentclass[border=10pt]{article}
usepackage{pgfgantt}




ganttset{group/.append style={orange},
milestone/.append style={red},
progress label node anchor/.append style={text=red}}

begin{document}
begin{ganttchart}[
y unit title=0.4cm,
y unit chart=0.5cm,
vgrid,
time slot format=isodate-yearmonth,
time slot unit=month,
title/.append style={draw=none, fill=RoyalBlue!50!black},
title label font=sffamilybfseriescolor{white},
title label node/.append style={below=-1.6ex},
title left shift=.05,
title right shift=-.05,
title height=1,
bar/.append style={draw=none, fill=OliveGreen!75},
bar height=.6,
bar label font=normalsizecolor{black!50},
group right shift=0,
group top shift=.6,
group height=.3,
group peaks height=.2,
bar incomplete/.append style={fill=Maroon}
]{2010-09}{2011-12}
gantttitlecalendar{year} \
ganttbar[
progress=100,
bar progress label font=smallcolor{OliveGreen!75},
bar progress label node/.append style={right=4pt},
bar label font=normalsizecolor{OliveGreen},
name=pp
]{Preliminary Project}{2010-09}{2010-12} \
ganttset{progress label text={}, link/.style={black, -to}}
ganttgroup{Objective 1}{2011-01}{2011-12} \
ganttbar[progress=4, name=T1A]{Task A}{2011-01}{2011-06} \
ganttlinkedbar[progress=0]{Task B}{2011-07}{2011-12} \
ganttgroup{Objective 2}{2011-01}{2011-12} \
ganttbar[progress=15, name=T2A]{Task A}{2011-01}{2011-09} \
ganttlinkedbar[progress=0]{Task B}{2011-10}{2011-12} \
ganttgroup{Objective 3}{2011-05}{2011-08} \
ganttbar[progress=0]{Task A}{2011-05}{2011-08}
ganttset{link/.style={OliveGreen}}
ganttlink[link mid=.4]{pp}{T1A}
ganttlink[link mid=.159]{pp}{T2A}
end{ganttchart}
end{document}

end{document}

I have this example from the pgfgantt documentation. But it doesn’t work.
I get this error:

! Package xcolor Error: Undefined color `OliveGreen’.See the xcolor
package documentation for explanation.Type H for immediate
help…. …ess=4, name=T1A]{Task A}{2011-01}{2011-06}