Mendel crossed two plants that were heterozygous for purple flower color. which genotypes could he have used to represent the cross?

The answer is two Pp genotypes.

If we imagine that plant has two alleles for the trait and if P represents dominant allele, and p represents recessive allele, then genotypes are:
PP - dominant homozygous, which contains both dominant alleles
pp - recessive homozygous, which contains both recessive alleles
Pp - heterozygous, which contains both dominant and recessive alleles