I managed to achieve this by creating rules in SLD . Passed CountryName as ENV parameter checking the values inside ogc:PropertyIsEqualTo. For default color i used same rule with ogc:PropertyIsNotEqualTo . The sample rule is as below
<Rule>
<Name>Selected Country</Name>
<Title>Highlight the country passed using ENV parameter</Title>
<ogc:Filter>
<ogc:PropertyIsNotEqualTo>
<ogc:PropertyName>ISO_CODE</ogc:PropertyName>
<ogc:Function name="env">
<ogc:Literal>country</ogc:Literal>
<ogc:Literal>USA</ogc:Literal>
</ogc:Function>
</ogc:PropertyIsNotEqualTo>
</ogc:Filter>
<PolygonSymbolizer>
<Fill>
<CssParameter name="fill">#BB6464</CssParameter>
</Fill>
</PolygonSymbolizer> </Rule> <Rule>
<Name>Default Colour</Name>
<Title>Colour All other countries in Diffrent Colour</Title>
<ogc:Filter>
<ogc:PropertyIsEqualTo>
<ogc:PropertyName>ISO_CODE</ogc:PropertyName>
<ogc:Function name="env">
<ogc:Literal>country</ogc:Literal>
<ogc:Literal>USA</ogc:Literal>
</ogc:Function>
</ogc:PropertyIsEqualTo>
</ogc:Filter>
<PolygonSymbolizer>
<Fill>
<CssParameter name="fill">#67D274</CssParameter>
</Fill>
</PolygonSymbolizer> </Rule>
Regards
Sudheesh