Both parents are heterogeneous, meaning that their genotype most likely appears as so - PW or Purple White.
If white is the recessive trait, then in a punnet square it would only have a 25% chance of appearing in the offspring while the other 75% would be purple. 50% being heterogeneous genotypes and the final two being homogeneous.
Therefore, the two purple flowered parents can produce a white flowered offspring due to the fact that they both are heterogeneous and hold the recessive trait of white.
Hope this helps!
