color contour line

Hello,
     Does anyone know how to make contour line with positive value a
color and that with negative value another color?
    I followed gsun02n.ncl second plot with
gsn_define_colormap(wks,"BlWhRe") , it doesn't work.
    Thank you very much for you help!
Mingxuan
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Received on Thu Dec 06 2007 - 10:59:37 MST