Some strains have a potential to produce purple pigment.
Some don't.
Some produce a LOT of purple pigment, and you can see it all the time.
Some only make a little and it is masked by the healthy green of chlorophyll.

Lack of nitrogen and cold temps make the chlorophyll break down and it is like pulling back a curtain to see what is behind it... PURPLE!!!
When you flush, you remove Nitrogen from the soil, and force the plant to use up whatever N it has left stored in its tissues, and not make any more chlorophyll. Eventually what is left breaks down and you can see other pigments that may be present.

No more fertilizer if you like the purple! Heck, no ferts a week from harvest anyway. Stop feeding, and start giving plain water when all your trichs are cloudy, right up until you decide it is harvest time.