The favoring of white flowers by natural selection would tend to increase the frequency of the white, p, allele in the population. However, since this is a recessive allele and peas are diploid, this would be slowed. Having two sets of chromosome per eukaryotic cell tends to act as a brake on change, especially for recessive alleles.
Work Step by Step
Review earlier material on genetics for help with this answer, especially where you can read how recessive alleles are hidden, in heterozygotes, from natural selection, and how having a diploid state means that natural selection acts on phenotype but not directly on genotype.