Can two purple flowered and yellow seeded parents produce offspring that have white flowers and green seeds?

Yes, as long as both parents are heterozygous with recessive alleles of white flowers and green seeds. In this case, the ratio of chance of resulting purple flower yellow seed to white flower green seed will be 3 : 1.

However, if both of the flowers are homozygous, which means they only have purple flower and yellow seeded allels, or even one is, the result is not possible, because we know that purple flowered and yellow seeds are dominant as both plants has these characteristics expressed.