Yellowing or turning white-is? Just the top of the plant?
Look up cannabis leaf deficiencies on google, go to pics and use those diagrams to give you a better sense of the illness your plant has. Learning to diagnose yourself is a big step towards learning your grow. Pic included to give you some scope of what to look for on google.
If it’s only on that one tall cola that is way above
The net it might just be light/heat burn
2 feet from light. From the top of the tallest cola. My light is a hps 800 watts dimmable light. I usaly turn lights down 250 an hour before lights out. In the morning I will turn it up to 600 watts. The soil I used had a low amount of nutrients I think like 5 8 8. I fed them 2nd week of flowering with a 9 15 13. I water them every 3 days half a gal each. The one on the back right corner is in a 3 gal bucket the rest are in 5.
Ok. I think it is a ph problem. I got a meter and checked it. Like 5 is what I was getting. Really acidic. Not sure how much lime I need to put on them in 5 gal bucket.
Looks to be a ph issue
I’d try a 1/4 cup to 5 gallons and ph the water to around 7 and feed them and check run off add more till you hit the 6-6.2 sweet spot
I had a cheap meter and checked it at several places in the bucket (5gal) . should I flush? I know I need lime, but don’t want to over do it.
Ph your water before you actually water the plant 
if your run off is reading 5 add water with ph of 7 the check run off again