Ah they look pretty health. You see those Cotyledon leaves, the little baby bottom ones, they are the food for the seedling. They go yellow as they are doing, then the plant needs feeding. If your growing in coco then that's the time to start introducing nutrients.
If you not in coco coir and your in compost or soil then there will be nutrients in there, there are no nutrients in coco coir.
Like I say, watch them, if the real leaves start to pale off then a light dose of veg nutrients. No PK boosters, no flower boosters.
You should avoid clear containers for plants. You could wrap them with something light proof. I wouldn't worry too much, they are looking well.
Just remember at this stage its easy to over do feeding so play it safe, less is more, gradually increase nutrients.
Your doing well.