This is my 1st grow and I’ve noticeD one of the top leaves going yellow . Im not sure if it’s a watering issue , nutrients or light burn . I’m watering about 200ml a day with 3ml /l advanced nutrients ( micro, grow, bloom & calmag) it’s 3-4 weeks old in coco soil.
Can we see a pic with the lights off mate? Really hard to see anything under blurple lights
Please see photo with grow lights off
@robh Cool, thanks man 
This could be a few things. A break in the leaf steam, but not likely.
Bugs is a possibility, thrips maybe, but id say it is most likely, just a genetic thing and its nothing to be concerned about mate.
Maybe someone else can offer more insight. But I don't think you will have any issues here The grow will carry on fine as it is
Looks a little like variegation, sometimes the leaf can discolor naturally sometimes, I have had it a few times and it's usually on the younger plants and fresh growth. It can also be a little bit of trauma to the leaf when it was emerging. Unless it continues to effect other leaves or worsens it isn't really an issue and will continue on its merry way. But as monkey mentioned it is a good idea to look at all the factors before ruling them out bugs, plant trauma, emvironment and nutrient imbalances would be my first go to items on the check list, if all of those are fine, put it down to variegation.
Looks like a harmless mutation.
My purple lemonade has something similar
I've had wind burn that looks the same too but would be most of the leaf
Lol just realised it was macky not monkey that replied sorry guys bit high and has Been a long day.
Looks like a genetic thing you can get cool funky looking variations of this that affects Evey leaf exactly 50/50 shouldn't be anything to worry about though 👌
Just piling on the comments. Definitely looks genetic, minor mutation, and she should grow out of it shortly. Just leave her as is and keep going forward Rob. I couldn’t help but notice you are using Advanced Nutrients and it sounds like you are using the 3 part soil nutrients. I also run Advanced Nutrients in coco but I find that the Sensi Coco formula works best for coco. It’s ph balanced specifically for the coco media. You will probably need to force the 3 part formula down to a lower 5.8 ph because I believe it’s made to automatically balance at 6.5 ph for soil use. Been a very long time since I’ve messed with the 3 part stuff. Hope I’m wrong actually.
Random acts of kindness make the world a better place
Just so your mind is at ease.
This girl has had that for about 3 weeks or so.
Nothing to worry about
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