I figured it out! Here's the code if anyone has the same doubt i had :-)
// draw the filled polygons
glPolygonMode( GL.GL_FRONT_AND_BACK, GL.GL_FILL );
glEnable( GL.GL_POLYGON_OFFSET_FILL );
glPolygonOffset( 1, 1 );
glColor3f( 0.0f, 0.0f, 0.0f );
renderData();
gl.glDisable( GL.GL_POLYGON_OFFSET_FILL );
// draw the wireframe
glPolygonMode( GL.GL_FRONT_AND_BACK, GL.GL_LINE );
glColor4fv( colour );
renderData();