They don't show any heat/light stress. I have some Lemon Kush Headband going currently that look just like your pics. I am also low to mid 60's for night temps. Running CXB 3590. I Think it has more to do with strain as only half of my strain are turning purple. Some of my bare COB's are...
I can tell you are having fun with them, the others not so much....haha. I am not entering that shit storm brother. They think you work for a LP, I have read a few of your posts in LED, it's real obvious you are a grower.
6031 is not a common grade, are you sure it's not 6061 ? 6000 series aluminum is expensive, look for 1100 series aluminum as it will have better thermal conductivity. It's way cheaper also, you should be able to buy 1/4" plate for what you're spending in 1/8" sheet in 6000 series. 3003 would be...
I do the same as you to confusing otherwise, weird some don't count the first week. Can't believe how much the COB's suck up the magnesium, they want more every feeding.
1800 watts cxb 3590, 36 at 50 watts each. Covers a 6ft x 8ft area. 1000 watts of timber, 6" x 90" long laser cut .080 alum sheet with 3 1/8" Dia holes to fit the cobs at 14" centers. 800 watts of diy with USA heat sinks.