I went to www.ask.com and input "how to paint a sink or a toilet" found this
I painted a sink(Kitchen) several years ago and used a product " tough as tile" I also have my kitchen tiles painted this way at least since ten years. It's an epoxy pain and self leavelin. No brush marks and it look like porcelain. Now, if you like to marble the sink take take either china paint and depending on the size you could cure it in the dishwasher.(hopefully)
An other option, just use acrylic paints and varnish at least 25 to 30 coats. I did this in a designer showcase and it still holds up. Naturally the last two otions are "high maintenance" for the owner because ideally they should always wipe the sink after each use(which I do anyway I am so compulsive LOL) I also gold leafed a sink and gave it lots of varnish and it still holds up also, but this is in a powder room.Don't forget to disassemble the sink whatever you do. I mean remove stoppers, metals etc.

I've painted tons of toilet seats, but never an entire toilet.

To paint a toilet seat, prime the lid with FLAT white paint. Paint your design with acrylic paints. Always use several thin coats as opposed to one or two thick ones. Let the seat dry at least overnight. Coat wih several coats of clear indoor/outdoor varnish. Don't use abrasive cleansers, just clean with a dampp cloth. I wouldn't advise painting the seat itself.

As far as painting the toilet itself, I would think that you could use deco perm enamel to paint designs on the outside of the bowl. The longer that paint cures, the more durable it is. Just be sure to clean the surface well with vinegar or alcohol and make sure that it's dry before painting.