Help needed - New leaves purple

Hi, I have three plants that are the same strain and a couple of days ago two of them started to darken on the center of the new leaf sprouts. The other one is in the same conditions and shows no signs of this.

I have the plants outdoors.

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Any help about what this might mean would be appreciated! Thanks!

Strain - Barney's farm blue cheese
Ph - 6,8
Soil - canna Terra professional
Time - 2 weeks

Not sure what to make of it, tested for P seems ok

Could it be genetics? I was under the impression the turning purple only occurred naturally when flowering (and temps too?)

Plant looks fine.....Probably genetics as you alluded to...