What is the phenotype ratio of the offspring of the plants in the Punnett square?

The heterozygous plants are purple, therefore purple is the dominant allele.

Assuming it is a single gene with only two possible alleles and the other colour is white; a cross between two heterozygous plants (Pp X Pp) should produce 3 purple and 1 white offspring. This means 3/4 will be purple.

1 offspring will have the genotype PP (purple), 2 will have Pp (purple) and 1 will have pp (white).