Change Cell on Value

I have searched and tried several, but either the whole list of cell goes the same colour or it does nothing
I have this

if ($data["Priority"]=="HIGH")
$record["Priority_css"]='background:red';

if ($data["Priority"]=="MEDIUM")
$record["Priority_css"]='background:yellow';

if ($data["Priority"]=="LOW")
$record["Priority_css"]='background:green';

Where an I going wrong please? in the List page:After record processed?
Cheers