My lettuces turned red

Above. Here are my trout lettuce plants, sown in April (still small because I keep picking the leaves, and they were very slow to start). They have all turned red. They grow in a trough about 10" deep, filled with organic potting soil. When the sun is overhead the shadow of the bigger pots behind the trough creates some shade. I say this because the reddening could be a reaction - protection against - too much sun.

Here is one of those same red lettuces one week after it was transplanted to a larger pot, filled with different potting soil. It turned green. It gets a little shade, especially in the morning, from an immature tomato plant.

I have transplanted four more red lettuces. The experiment continues.