A crack to be repaired must be absolutely dry. Empty a toilet or tank, wipe it dry, then let it dry completely; you can hasten drying with a hairdryer.
Use a two-part epoxy adhesive that takes 30 to 60 minutes to set, not the fast-setting, 5 minute kind; it doesn’t stand up well to water.
Following the package directions, mix the two components catalyst and hardener and use a toothpick, matchstick, or the like to fill the crack. Wipe off the excess; let it cure for 12 to 24 hours.
To hide the crack, use an appliance touch-up paint, available in a variety of colors, sometimes with a tiny brush included. Follow the label directions.
A chip can be covered with porcelain enamel repairer, which can be bought at a plumbing-supply house.