Eggs are white in color until the last moment before laying, when the hen puts the "bloom" on. The color or the bloom as it is called is basically a dye and each hen has different amounts of it in her glands. The more she lays, the lighter the color on the eggs becomes. If you are sure you have only one hen in this cage, it is possible she laid two eggs in one day, or laid before she had time to manufacture more "dye" and hence, the egg was laid white.