If purple flower color is dominant over white flower color, how can two purple flowered parents produce white flowered offspring

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!

the tricuspid valve is present between right atrium and right ventricle