Seed are reported to be viable for up to a year or more. All species and hybrids of bougainvillea can give rise to mutations or bud-sports and give us an entirely new cultivar. It appears that in countries where bougainvillea are native, mutations occur more readily than in the temperate zones. This is why there are many variegated bud-sports of Elizabeth Angus and Mrs. Eva . These two cultivars are the most often grown varieties by our local nurserymen.
Pollution is also another factor that gives rise to variegated bud-sport.
Various form of pollution especially chemical pollution cause the genetics of the plant to become unstable and hence mutate. Most of these bud-sports are discovered accidentally by nurserymen. There is little 'in-depth' resources on the web for Bougainvillea. If you know of a site, let me know. This page doesn't provide links, just a listing of various writings concerning bougainvillea.