Variegated leaves are those that are not completely green in color but there are areas on the leaves that are white in color. This is due to the absence of plastids or mutant plastids that cannot produce the chlorophyll pigment essential for photosynthesis.
If a variegated leaf were to be soaked in alcohol to remove all the pigmentation and then an iodine test were to be performed, the areas on the leaf that were originally green will show the presence of starch while the portions that were originally white will not give a positive result.
Photosynthesis is essential for the production of monosaccharides and their storage in the plant as starch. The iodine test can only detect starch. In areas of the leaf that are white and where photosynthesis does not take place monosaccharides are not produced which in turn does not allow their storage as starch.