What could it be? Nutrient deficiency?

Your mandarin plant is highly deficient in potassium btw (leaves yellowing from the outside with brown spots)

CalMag deficiency and slight deficiency in flowering nutrients. (potassium and phosphorus)

Calcium deficiency. Definitely not "rust" or due to cool night temps. Absolutely no need to "flush".........add a cal/mag supplement to your line up, or increase the dosage if already using a cal/mag supplement.

Thats just a lil bit of rust rust-colored spots on marijuana leaves indicate a fungal infection or a deficiency in magnesium or calcium. Nutrient burn is also a possible culprit. i dont think its a fungus, those you can rub off.. mayb its a lock out, she had enough for now i think. flush her with just water so it wont get worse good luck

Could be phosphorus deficient, or could just be from cooler night time temps. I've found below 70F that purple leaves can occur and it's ok/normal. Mine do this when the temp drops below 68F at night. Hasn't effected my outcome to this point on my grows and looks cool to me, so I don't worry about it. Just my thoughts. Best of Luck! Happy Growing!