Variegated leaves have areas that are white in color instead of being green. This is due to a lack of plastids that contain chlorophyll; without chlorophyll photosynthesis cannot take place.
The areas of the leaves which are green produce monosaccharides through photosynthesis and this is stored as starch.
If a starch test is being performed on a variegated leaf, the white areas can serve as the control areas. The starch test would be used to determine if there is a proper production of starch in the green areas and to test if the conditions under which the plant is kept are appropriate for this. The white areas would give a negative result no matter under what conditions the plant is kept. The result obtained from the green areas can be compared against this.