There could be a number of reasons for seedling to turn yellow:

1. The most probable cause is over watering, although, confusingly, under watering can also be a problem. With your finger, feel the soil an inch or so below the surface and only water if it feels dry.

2. Too strong light from sunlight or artificial light can also be a cause.

3. Seedlings do not usually need fertilizer. In older plants you may have a nutrient deficiency.  If you have applied a chemical fertilizer then you may have “fertilizer burn”. Use an organic fertilizer instead.

4. Moist conditions can also lead to a very common issue -- “damping-off’( a fungus disease). Provide better ventilation and less water.

Courtesy of NYBG Plant Information