Edit: Sorry, posted info on plants in general, not seedlings. Will keep the rest there incase it's helpful.
But, my seedlings usually start this way and go green in a few days.
Later life red petioles.
All of my red petioles and purple stalks have been caused by deficiencies, which was actually caused by my LED being way too close to the plants. I corrected those deficiencies by adding CalMag+, which stopped new red petioles, and faded some of the red ones back toward green.
If you are running LED and also appear to have leaf damage and you can't figure out why, check the height of your LED. My issue had me chasing deficiencies for a while. It was maddening. Everyrhing was perfect. pH, nutirent regimen, water cycle, light cycle, humidity, temps, everything. Once I pumped up the deficient nutrients, which were likely locking out some others, like phosphorous, my auto plants developed no new damage, but instead went perpetually droopy.
Fixed that after a half week of waiting for a change by using a less powerful LED. Raising the light would have worked too but I was out of vertcal room.
I can only confirm this was the case when reddening starts as only the top half of the petioles. It was very obvious, bright red up top, green on the bottom, a razor thin line between. Eventually they went fully red and my stalks started doing the same. Red on one strain, and a deep purple on my autos.
My grow journals below show what happened to my plants; what I saw, what I did, and what I finally determined the root cause to be.
My plants are all healthy and happy now, even the purple stalks are greening again.
This happened to me in two grows under Mars Hydro Mars II 700W lights in a pretty small space at about 14" from canopy on smaller plants (< 36" tall).
