app/design/adminhtml/default/default/template/widget/grid.phtml

_column->getCssProperty()

Mage_Adminhtml_Block_Widget_Grid_Column::getCssProperty

$this->getRenderer()->renderCss();

Mage_Adminhtml_Block_Widget_Grid_Column_Renderer_Abstract::renderCss

$this->setTemplate('path/to/your/grid.phtml');

$this->addColumn('code_here', array(
        'header'    => $this->__('label here'),
        'index'        => 'code_here',
        'column_css_class' => 'has_dynamic_class'
    ));

Event.observe(window, 'load', function() {
    $$('td.has_dynamic_class').each(function(elem){
         var value = $(elem).innerHTML;
         var class = calculate class here depending on 'value'
         $(elem).addClassName('class');
    })
});

public function getRowClass($row)
{
    return $row->getId() == 123 ? 'selected-row-class' : '';
}

$this->addColumn('my_col', [
    'header' => 'Col Name',
    'index' => 'my_col',
    'column_css_class' => 'my-specific-col'
]);

.selected-row-class.my-specific-col {
    background-color: pink;
}