Although I've never heard of ceramic heads, I use artificial polymer heads, and you'd be better off getting a small set of oil base paints and hand painting them with a nice camel-hair brush. Could you use a spray paint? I guess you could, but quite a few ducks have multi-colored bills requiring some detail, detail you won't get with a spray can. Also an airbrush set isn't too expensive, and worth every penny. Not just for taxidermy, but for painting decoys etc.
You will need to run a wire from the necking to the head, that's the part that surprises me with your ceramic head, which more than likely isn't ceramic, but is a hard polymer head as well. Don't think I'm calling you a liar, just never heard of it. However, that doesn't mean that they don't exist. I could be wrong, and am missing out on the latest and greatest. Most people drill a small hole in the polymer head, and either superglue or hot glue the wire from the necking to the head. Please don't try to glue the scalp to the artificial head, instead, use caulk, it is much more forgiving, and doesn't set up too quickly that you cannot manipulate the skin to the head. If you have any other questions please feel free to re-post, and I'll try to help.
One other thing, Who is the manufacturer of the artificial head you're using? Joe Ferebee, Tony Finazzo, and Frank Newmeyer, are some examples of artificial bird head guys. All of these are polymer type heads.