ClutterInputMethod does not have the color info in preedit
libmutter-clutter
has clutter_input_method_set_preedit_text()
but it does not have the color information.
IBus
has ibus_engine_update_preedit_text()
and the argument IBusText
has the color information.
https://github.com/ibus/ibus/blob/master/ui/gtk3/pango.vala#L23
Probably I think clutter_input_method_set_preedit_text_with_attributes() or something is needed.
gtk_text_input_send_preedit_string() has no color information: https://gitlab.gnome.org/GNOME/mutter/blob/master/src/wayland/protocol/gtk-text-input.xml#L244