Chipp's tutorial only works with plants that have empty subsets.
Solidgrowth means that each plant is different from the previous one, be it in size, shape AND colour. Some plants have embedded colour variation, like the coconut that turns red sometimes, or the Mexican palm that has grey dead leaves sometimes. And there is not much yu can do about it. The only  possibility is to keep generating new plants in the plant editor, until you get one that has no or minimum colour variation, and then save this one as a new species. Sometimes, it will work, but sometimes the variation will come back.