The two biggest factors for light green/yellowing leaves during seedling phase is over-watering or nutrient deficiency. Nutrient lock-out is also possible but highly unlikely unless you are over-feeding your plants. If your soil is wet, let it dry out almost completely between waterings, you can tell when they need water when the pot feels light or the plant is beginning to wilt. This will not harm your plant as long as you don't take it to the extreme. If over-watering isn't an issue, then you can feed your seedlings with a low dose of high N fertilizer, I use Fox Farm Grow Big (6-4-4). I would not feed more than once a week so you don't burn your plants. Increase the dosage as the plant grows and responds to the water/feeding schedule. Don't expect immediate results, it will take some time for your plants to recover, but if you have patience, you will be greatly rewarded for your efforts.