The most proper fix is to replace the tile.
If that's not in the cards, you can use colored markers (the ultra fine tip variety you'd find at an artists' store). Do not color in the chip like you would with a drawing. You need to lightly tap many, many tiny dots onto the tile with space between the dots. Start out with a light smattering of dots and slowly add dots to darken the color. Take a step back after you've put some dots on and see what it looks like from several feet away. Be artistic. Add a couple dots, stand back, & evaluate again. Use more than one color to get the best match.
