It's my first time with Pak Choi (Bok Choy) and so I don't know what seedlings normally look like. But should they be this varied in shape and colour?
I've grown Pak Choy a few times and I get a bit of colour variation. I've found, with other plants as well as pak choy, that it usually has to do with sun exposure. Perhaps a shadow is being cast across a few of them for parts of the day? I have a coriander plant, for example, that has very pale leaves in the part that is behind a pole and the greenest of green leaves in the part that gets good sun!
I had it few times, bought in a nursery, and they were exactament so. the guy explained that part of these were behind a pole of the greenhouse so got less sun.
Then I have grown from seed, it was impossible to put them all in the same position and the result was the same. But in fact, places to stay, they have grown regularly, beautiful and healthy.