Gdk.color GTree.column ?

Hi,

is it possible to have tree columns
containing GDK colors, as this is possible
with basic types (int, string, ...) and general
Caml values (caml) ?

I'd like to write something like :

       let cols = new GTree.column_list in
       let text = cols#add string in
       let count = cols#add int in
 >>    let bg_color : Gdk.color GTree.column = cols#add ???
       ...
       let model = GTree.tree_store cols in

       let view = GTree.view ~model:model () in
       let cell_text =  GTree.cell_renderer_text [] in
       let col_text =
	let col =
	  GTree.view_column ~title:"Text" ()
	    ~renderer:(cell_count, ["text",count]) in
 >>	col # add_attribute cell_text "background-gdk" bg_color;
	col in
       ...

What would you put in place of ???  ?
If I put 'caml', then I can put GDK colors in the model,
but column view 'col_text' can read them because they are
seen as 'caml' values instead of GDK colors.

---
Sébastien