White flowers: WW
Red flowers : ww

Homozygous plants have one type of allele, while heterozygous plants have 2 different alleles while the dominant one(uppercase) shows and the recessive one(lowercase) doesn’t show, so since both of these plants are homozygous, both alleles are the same.

Ex: Homozygous: WW , ww (dominate one shows, recessive one shows)
Heterozygous: Ww ( only dominate shows)