Problem with the leaves. Im at week 4 of flower growinf photoperiodic white widow in soil medium and im feeding every 2 days 1 liter of solution per plant

It is no problem......the plant is getting closer and closer to its end. So its "normal" you see leaves change. If you think it is too early for letting her "eat her leaves"........you can increase your ec. As it looks like a dificiency of iron, nitrogen or cal/mag. If this strain still takes 6 weeks to finish, you should change nutrition. IT also might be that a strain naturally changes leave colour without any dificiency. To me your plants look fine

They are not so bad as you think. They show a little light stress and i think you shoud give here more water and nutrians. The ppfd is a little hight at this stage. 😀

You may not be feed enough to be honest

High vpd can reduce potential photosynthesis ceiling per day. Which could be exacerbating what you see int eh canopy, despite 900 ppf (umol/s PAR) central reading not being too hot. PPFD is not a single point measurement, for what it's worth. humidifiers are cheap to run. I'd suggest one with a wick, which is more expensive to maintain, but then you don't get the white film that the 'atomizer' type humidifiers cause. End up smoking layers of residue, lol. try turning exhaust down a bit? usually recommendation suggest more than necessary.. a slower gas exchange could improve humidity levels, but to be certain of no other ramifications would need to ensure it doesnt also impact co2 levels. If it's currently high-powered, probably safe to drop a little. Obviously adding a humidifier is a better option. fix the watering habits as other said... a better vpd and proper watering might fix everything else. if the chlorosis is bottom-up, add a little N to your fertilizer. can't see if it's originating at the top or bottom in the pictures. If starting at the top, i'd wager it's more about vpd and light levels not jiving.. the purple is nothing... that is fall colors.. could be exacerbated by night time temps near or below 18c.

Purple is ok. Paleness is not good at this stage. As Hashy said, watering X amount every Y days is not the way to do it...........water thoroughly and deeply then do not water again until the top 4cm have dried out. Water each plant to its individual requirements, there is no "one size fits all" when growing cannabis in soil.

Normal. Its moving to show its fall colors.

They don't look that bad to me, one thing you mention is watering every 2 days with 1L. With soil I find it's better to thoroughly soak the soil slowly until I get a bit of runoff rather then giving a pre determined amount. Example I have 2 plants in 12L pots they require let's say 4-5L before I get runoff, one plant may need watering 3-5 days later but the other may need watering 5-7 days later. Its best to water when they need and a thorough watering.