Using below code i'm getting list of filterable attributes

$layer = Mage::getModel("catalog/layer");
$attributes = $layer->getFilterableAttributes();

I can't able to set filters and get it's sub filters. example i have below 2 filterable attributes,

Size
 28 (1)
 30 (2)

Color
 Green (2)
 Red (1)

On selecting Size 28, my sub filter is

Color
 Red (1)

So i'm trying to achieve below,

• Get all available attribute filters initially (got this using getFilterableAttributes())

• Set filter programatically and get it's sub filters as like above example