The genotypes of the parents are Aa (heterozygous) and aa (recessive homozygous).
For example:
If the purple flower is dominant phenotype, then A is a dominant allele for it and Aa is a genotype which will give the purple colour.
In this case, white flower is recessive phenotype with aa genotype.
If we cross Aa x aa
The offspring is going to be Aa Aa aa aa (half purple and half white)