Purple color under leaves

From what i can tell plant is growing really fast,and as i said still some pigmentation problems here and there but everything seems fine.I decided not to let temps down under 18°C .Monday they will send me the new humidifier so hope for the best.I should mention at start that i germinated from seed in soil with Biobizz all mix wich some said its a bit 'hot' for seedlings and if this is true thats why these problems and that's why now plat seems stabilized since biobizz is perfect for grown plants like mine week 3 alrdy.Thanks for help to everyone

Just in case you think I'm fucking around...here's a clue about that. Personally, I associate purpling mostly with light, calcium, phosphorus issues (whether it be plants at end of flowering, or suited for cold areas, whatever). "Anthocyanins are a type of natural pigment that have high potential for development and utilization in regions like food, pharmaceutical, and cosmetic industries, with nitrogen and phosphorus starvation possibly promoting their accumulation in grapes."

Very green, dark, purple is often related to magnesium, calcium and phosphorus.

Just to clarify, calcium because it's a buffer and can aid electrical balance, uptake of things, firstly...but it's also enormously common and sort of "safe". A little bit of phosphorus, of the minerals it's pretty much the most sought-after by living organisms. Phosphorus is the most "recycled" element and is WAY more concentrated in living organisms than background levels. About 100x more than carbon.

I think you might need a bit of calcium/phosphorus considering the look of that (very dark, purpling underside), *maybe* carbs/sugars.