How big are the chips? If they are just small chips, you can first, clean the area with soap and water. Let it dry completely, then just use some primer and paint, to match the tile color (don't totally fill the chip, just use enough to match). Then use some epoxy to seal it. However, if the chips or cracks are to big, you'll have to replace the tile.
If the chip is near an edge, grout can sometimes be used...this works better on more "rustic" type tiles such as tumbled marble etc. other wise as Dan's mentioned above paint to camouflage or replace the problem tile.
Check out this article I wrote on repairing cracked and broken tile.
I have had good luck with the little jars of paint made for model planes and cars. Wide array of colors, so I have always been able to match the tile pretty well. You can build up layers of paint, although I have never repaired a chip larger than a quarter coin.
Nail polish works well, and easy to colour match