The hypocodyl - the region below the codyledons, it will stretch until the leaves receive blue & red light and then this creates a hormone change which will cause a number of photomorphogenetical changes, like growing of true leaves, stem thickening instead of elongation, adventitious root formation etc pp. But it's important to understand that hormones travel slowly through the sap, so there's always a delay.... everything with plants is slow so that's why just letting them grow is oftentimes best... they can adapt to alot of various conditions anyway
I think your seedlings will grow fine though I dislike blurple, just 2 colours is not really good... but as you can see the upper 2 nodes are already fairly close... and the stem will thicken up more later...