You may be worrying a little too much, all seedlings have different colour and lustre of leaves alters with changes in the weather.
On the other hand it is hard to say for sure without seeing the problem. I would wait and see, although you have not mentioned watering and I wonder if you may be overdoing it. Seedlings need almost none, maybe every four or five days unless there is brilliant sun and lots of air moving. Too much water is often worse for seedlings than too little. Wait until the compost is slightly cracking away from the edge of modules or pots before watering again.
Too much water can flush out nutrients unless it is a really good organic compost. But seedlings need hardly any nutrients and you should not notice a yellowing from lack of nitrogen until plants are well grown, almost ready to plant. Temperature affects rate of growth more than leaf quality, in the ranges you have mentioned. My greenhouse was 3C last night and seedlings look fine.