Dirt Gently's Horticultural Adventures: The purple menace

So my lovely, vigorous moonflower seedling has been gradually turning purple over the last several days. What's up with that? The purpleness started with the stem and crept its way up into the veins of the seed leaves. The leaves themselves are now a mottled with yellow, green, and purple-black areas instead of the healthy green of a couple days ago.

February 28: Lower stem purple but otherwise healthy looking

March 1: Seedling still looks healthy ...

March 1: ... and so I'm not worried yet

March 3: Now worried. Purple stem, veins. Leaves purple-black, yellow. Sad trombone.

Even though some other seedlings also have purple stems, and true leaves appear to be emerging from this moonflower seedling (see below), the seed leaves look pretty unhealthy. Am I worrying too much?

Moonflower crotch shot. Wonder what kind of search engine referrals this will bring?

Some googling leads me to believe that this may be due to impaired phosphorus uptake and the resultant increased concentration of anthrocyanins in the plant due to restricted transport of sugars around the plant. Possible culprits include restricted root growth (compacted soils), too-cold or too-moist soils, too much light, or a shortage of micronutrients (B, Fe, Mg) (Why are corn seedlings turning purple?, Purple pepper leaves, Help! My seedlings are turning purple, what do I do? Function of phosphorus in plants). Given that it's pretty warm in the propagator, the soil is moist-but-not-wet, and there's a rootlet poking out of the drainage hole at the bottom of the pack, I am leaning towards restricted root growth. Time to pot it up, I guess ...

Hopefully the toilet paper rolls are deep enough for the moonflower seedlings in Dirt Gently's Nursery for Slightly Fungus-ridden Toilet Paper Rolls.

In the same plastic 4-pack as the purple moonflower, a couple of "chinese spinach" seedlings seemed to have succumbed to damping off, or maybe they were just weak to begin with. I will have to keep an eye on things. I wasn't so concerned about how purple they were because that's how they came up.

February 27

February 28

March 1: 2 weak-looking seedlings top centre

March 3: Weak seedlings die

March 3

It looks like even more little lithops seedlings have appeared overnight. They are delivering 100% concentrated happy like champs.

Meanwhile, in Dirt Gently's Nursery for Slightly Fungus-ridden Toilet Paper Rolls, the seedlings keep on coming up and the white fluffy fungus seems to have mostly disappeared. You can see that the spinach and bok choy seedlings are starting to develop true leaves. Like the moonflower, the stems of some of the seedlings are also turning purple, but it can't be because of restricted root growth: the seedlings are tiny, and there's at least 3" of medium in the TP rolls.