Genetically it must first have the ability to do so, comparable to having the correct drivers loaded on a computer it must first know how to produce anthocyanin or one of the 350 or so known types.
1.Genetics
2.Temp
3.PH
You have your ph at 5.9
Nitrogen is a mobile nutrient, meaning it can be redistributed where needed for new more "important' growth like flowers
This makes nitrogen and potassium hard to uptake through rootzones, because nitrogen uptake is restricted the plant's veg growth is starting to yellow like yours.
it's not that you need to bring them out, you need to force the plant to change for its own survival.
Anthocyanin (purple pigment) to replace chlorophyll the environment needs to provide more PAR (photosynthetic radiation) in the blue/UV regions as the photoreceptor CRY is responsible for harvesting energy from the blue/UV wavelengths in order to also produce the anthocyanin.
growdiaries.com/diaries/156342-grow-journal-by-exbeginner
add 1-2 tablespoons of baking soda (fast acting) per gallon to watering to ph up to 6.5-7