Geoserver SLD variable substitution attribute based color assignment in wms call URL . Simpley add &env=a:000066;b:6666ff to the end of your wms call and you will have dynamically changed the zone "a" color to 000066 and zone "b" color to 6666ff

geoserver_sld_variable_sub

<?xml version="1.0" encoding="UTF-8"?>
<StyledLayerDescriptor xmlns="http://www.opengis.net/sld" xmlns:sld="http://www.opengis.net/sld" xmlns:ogc="http://www.opengis.net/ogc" xmlns:gml="http://www.opengis.net/gml" version="1.0.0">
  <NamedLayer>
    <Name>Study Zones</Name>
    <UserStyle>
      <Name>Study Zones</Name>
      <Title>Default zone style</Title>
      <Abstract>Study Zone style</Abstract>
      <FeatureTypeStyle>
        <Name>Study Zones</Name>
        <Rule>
          <Title>Zone A</Title>
          
          <ogc:Filter>
            <ogc:PropertyIsEqualTo>  
              <ogc:PropertyName>zone_name</ogc:PropertyName>    
              <ogc:Literal>a</ogc:Literal>  
            </ogc:PropertyIsEqualTo>
            
          </ogc:Filter>
          <MaxScaleDenominator>50000</MaxScaleDenominator>
          <PolygonSymbolizer>
            <Fill>
              <CssParameter name="fill">
                #<ogc:Function name="env">
                <ogc:Literal>a</ogc:Literal>
                <ogc:Literal>FF0000</ogc:Literal>
                </ogc:Function>
  
              </CssParameter>
            </Fill>
            
          </PolygonSymbolizer>
        </Rule>
        
        <Rule>
          <Title>Zone A</Title> 
          <ogc:Filter>       
            <ogc:PropertyIsEqualTo>    
              <ogc:PropertyName>zone_name</ogc:PropertyName>
              <ogc:Literal>b</ogc:Literal>    
            </ogc:PropertyIsEqualTo>
          </ogc:Filter>
          <MaxScaleDenominator>50000</MaxScaleDenominator>
          <PolygonSymbolizer>
            <Fill>
              <CssParameter name="fill">
                #<ogc:Function name="env">          
                <ogc:Literal>b</ogc:Literal>
                <ogc:Literal>0000ff</ogc:Literal>
                </ogc:Function>      
              </CssParameter>
            </Fill>
            
          </PolygonSymbolizer>
        </Rule>
        
      </FeatureTypeStyle>
    </UserStyle>
  </NamedLayer>
</StyledLayerDescriptor>