I have heard it is a mixture of two things... The phenotype of the purple strain, and the temperature. Apparently colder temps bring out the purple color for some purple strains.
Yes he is correct. One i would purchase a well known seed company's seeds of a purple variety and grow them in ideal conditions with low temps. Also might need to go threw many seeds to find an ideal phono.
Lower temperatures "restrict" the uptake and utilixation of certain nutrients, resulting in a color change due to a defficiency. Not a true form of "purple" buds.. IMO.
True.. purp's should express the colour without abuse.. IMO.