Can anyone explain gene complementation to me?
Such a phenomenon is possible via Dominant Negative transmission. In such cases, although p is the recessive allele relative to P, such that a heterozygote Pp plant will have purple flowers, as will a homozygous dominant PP plant, a homozygous recessive pp plant will also produce purple flowers. Dominant negative or antimorphic mutations occur when the gene product adversely affects the normal, wild-type gene product (i.e., white flowers) within the same cell. This usually occurs if the product can still interact with the same elements as the wild-type product, but block some aspect of its function. Such proteins may be competitive inhibitors of normal protein functions.
