See the explanation below.
Work Step by Step
Take note that the white sectors have formed near the edge of the red colony when the ADE2 gene arises and is expressed from its normal chromosomal location. When the ADE2 gene is moved to a location near a telomere, it is then packed into hetero-chromatin and is inactivated in most cells, giving rise to colonies that are going to be mostly red. This explains why they form near the edge in the first place.