Archive file with this example: ComboBox-Example004.tar.gz. Be sure to read the file README.txt for further instructions.
Individually programmed cell renderers are seemingly not possible with Haskell, as the Gtk extension methods are not included in the Haskell interface to Gtk.
However, it might be possible to create a ComboBox with a two-column liststore as data model, that uses a CellRendererPixbuf to display the colour and a CellRendererText to display the colour name.