functions attached to manage_pages_custom_column
get fired for every custom column, you have to check the name of the column within the function to only output data for that specific column:
add_action( 'manage_pages_custom_column', 'AddColumnValue', 10, 2 );
function AddColumnValue( $column_name, $post_id ) {
if( 'thumbnail' == $column_name ):
echo 'Header Image data here';
elseif( 'cssColor' == $column_name ):
//code goes here for second column
endif;
}