mapnik/tests/visual_tests/styles/raster_symbolizer.xml

<Map background-color="white" srs="+init=epsg:32630" minimum-version="0.7.2">

  <Style name="two-value">
    <Rule>
      <RasterSymbolizer opacity="1">
        <RasterColorizer default-mode="discrete" default-color="#000000">
          <stop value="0" color="yellow"/>
          <stop value="26" color="purple"/>
         </RasterColorizer>
      </RasterSymbolizer>
    </Rule>
  </Style>

  <Style name="ramped">
    <Rule>
      <RasterSymbolizer scaling="near" opacity="1">
        <RasterColorizer default-mode="linear" default-color="#00000000">
          <stop value="0" color="red"/>
          <stop value="26" color="green"/>
         </RasterColorizer>
      </RasterSymbolizer>
    </Rule>
  </Style>

  <Layer name="dataraster" srs="+init=epsg:32630">
      <StyleName>ramped</StyleName>
      <Datasource>
          <!-- get stats on this file with gdalinfo ../raster/dataraster.tif -stats -->
          <Parameter name="file">../../data/raster/dataraster.tif</Parameter>
          <Parameter name="type">gdal</Parameter>
          <Parameter name="band">1</Parameter>
      </Datasource>
  </Layer>

</Map>