Since the parents are heterozygous each of them will produce 1/2 gametes with the purple allele and 1/2 gametes with white allele.
When these will combine after fertilization there is 25% chance that the offspring will be homozygous with dominant allele, 50% chance that the offspring will be heterozygous and 25% chance that the offspring will be homozygous with recessive allele.
The homozygous with dominant allele and the heterzygoys offsprings will express the dominant purple trait (25%+50%) and only the homozygous individual with recessive allele will show the white colour.
