It looks to be a css priority case. Try using:
color: gray !important;

Refer http://stackoverflow.com/a/1637380/385377

You just need to replace "Total" with "total" … as $col["name"] is "total" with small t.
!important is not required.

$f = array();
$f["column"] = "total";
$f["css"] = "'color': 'red'";
$f_conditions[] = $f;
$g->set_conditional_css($f_conditions);

Hi Abu
I insert this code (indented) in your datarangepicker.php but is not working

$col = array();
$col["title"] = "Total";
$col["name"] = "total";
$col["width"] = "50";
$col["editable"] = true;
$cols[] = $col;

$col = array();
$col["title"] = "Closed";
$col["name"] = "closed";
$col["width"] = "50";
$col["editable"] = true;
$col["edittype"] = "checkbox"; // render as checkbox
$col["editoptions"] = array("value"=>"1:0"); // with these values "checked_value:unchecked_value"
$col["formatter"] = "checkbox";
$cols[] = $col;

$f = array();
$f["column"] = "Total";
$f["css"] = "'color': 'red !important'";
$f_conditions[] = $f;
$g->set_conditional_css($f_conditions);

$grid["rowNum"] = 10; // by default 20
$grid["sortname"] = 'id'; // by default sort grid by this field
$grid["sortorder"] = "desc"; // ASC or DESC
$grid["caption"] = "Invoice Data"; // caption of grid
$grid["autowidth"] = true; // expand grid to screen width
$grid["multiselect"] = false; // allow you to multi-select through checkboxes
$grid["export"] = array("format"=>"pdf", "filename"=>"my-file", "heading"=>"Invoice Details", "orientation"=>"landscape", "paper"=>"a4");

Thanks in advance

Ok.
It was a my error.
Now all ok.
Tks

Thanks for updating.