When you accidentally drop a heavy pan into a cast iron or steel sink, you're likely to chip the hard enamel surface. I know the sinking feeling, having chipped one myself hardly a year after installation. Puns aside, it's fairly easy and inexpensive to repair chips so they're almost invisible. You can find two-component epoxy (catalyst and hardener) in the adhesive section of most hardware stores and home centers. It's usually available in a variety of colors. If necessary, two colors can be mixed for a more precise match.
First, scrub the chipped area thoroughly with a sponge and soapy water. Then rub 400- to 600-grit “wet-and-dry” sandpaper over the damaged area to remove dirt and rust, as well as rough up the chip so epoxy will stick to it. Next, mix the two epoxy ingredients according to label directions.
Use a wooden matchstick or small brush to fill the chip. If the chip is deep, apply the material in several coats, and don't forget to allow for the drying time specified on the label. Once the repair is complete, wait 24 hours before you use the sink, and don't scrub that area for seven days.