I think it's a lack of nitrogen and not magnesium. make sure your soil is not too wet and start fertilizing I can recommend Biobizz they do a good job check out my OBG Kush Report. Nitrogen gives the plant its strong green (formation of chlorophyll) and gives it the opportunity to gain energy from light (i.e. to carry out photosynthesis).
Stay high Mrhashman✌️