Tomato seedlings turning yellow can be caused by several common factors including overwatering or underwatering, nutrient deficiencies (especially nitrogen or micronutrients like magnesium or iron), insufficient or excessive light, and transplant shock. Overwatering causes root problems like root rot which prevents nutrient uptake, leading to yellow leaves. Lack of essential nutrients or improper fertilizing can also cause yellowing, especially if seedlings run out of the nutrients in the seed. Seedlings also need enough bright but indirect light; too little light causes yellowing due to low chlorophyll production, while too much direct sun can scorch leaves. Natural yellowing of the first seed leaves (cotyledons) can also occur as the plant grows new true leaves. Key reasons and fixes:

• Water: Keep soil moist but not soggy, avoid overwatering (let soil dry slightly before watering again).
• Nutrients: Fertilize lightly with balanced, water-soluble fertilizer after the first true leaves appear.
• Light: Ensure 6-8 hours of bright, indirect light; avoid scorching sun.
• Transplant shock: Handle roots gently and give time for recovery if recently moved.
• Natural progression: Some early leaf yellowing is normal as seedlings grow.

So, addressing watering habits, providing adequate nutrients, and ensuring proper lighting conditions are the best steps to prevent and correct yellowing of tomato seedlings.