Local Harvest Gardening

I've noticed that the last few times I've started brassicas the leaves start to turn yellow and purple once they have 2-4 true leaves. Any idea why this might be?

I've got them under lights
I use soil blocks and have them in a seedling tray (like from the garden shop, but more rigid, and I can water in the tray rather than overhead water)
I used the sterile soil mix - I watched the videos on worm castings etc. after I started these ones
They haven't had any water stress as far as I can tell.

Any ideas?

It's likely they're lacking in nitrogen. Good you're not overwatering.

At this point, you can apply vermicompost around the plants and water it in. Or, if the roots have pushed through the bottom, put the tray on top of another tray that has 1/2 of vermicompost or any compost for that matter.

They'll turn around quite fast.

Or, you might want to transplant them into your garden already even if they're a bit skimpy. They'll recover really quickly.

The vermicompost in the tray did the trick, and quite quickly as you suspected! Thanks!