colour change on leaf

DutchDoobie is right, I'd give him the chosen answer to be fair

It's all good, Tee. It could be one of two things: the start of a little nutrient burn, or the start of a calcium deficiency. I suggest watering your plants with just pH'ed water for a few days while keeping an eye on the tips of your leaves. If the browning is only happening to the tips and edges of the leaves, it's nutrient burn. If the brown spots start going on any part of the leaf, not just the edges and tips, it's a calcium deficiency. For this, you just need to hit it with some CalMag and she should be good to go. Good luck!

Ok, calm down... you're NOT going to lose the plant! Two things here... I think what you're seeing is the very start of a calcium deficiency (the spots) and that's probably brought on by a nitrogen toxicity (dark green/brown leaves) which has caused a lockout of calcium. So I would flush her out - 9 gallons of pH'd water through each pot - but in the last gallon, give her 1/2 strength nutes. Then wait til she dries out completely before watering her again. I would also, going forward, reduce the nutes that have high nitrogen levels in them... plants in bloom (which this one is about to be in if she's not already) need SOME nitrogen but not a lot. And I would also tell you to start some bloom nutes as she's going to be needing them! You're ok... you caught it in time! You might see SOME progression in the symptoms of the cal deficiency but they will stop. The damage, however, will not be repaired but you don't have to worry about that. Good luck!