Hello. The purple color of the stems and cotyledons, as well as the lighter color of the true leaves of the tomato seedlings, may be caused by a number of stress factors, such as: lack of nutrients, limited space for root development, possibly low substrate temperature, etc. In this case you may consider transplanting the seedlings into larger pots or trays. After transplanting the seedlings, it is recommended to use a special fertilizer formulated for seedlings and young plants, such as , which stimulates the vigorous and healthy growth of the plants, including of the root system.
Throughout the growing season, for the prevention of nutrient deficiencies and for the development of tomatoes, it is advisable to use an organic fertilizer such as . A complete fertilization plan can be found in our article: Tomatoes - fertilization at different stages of crop development.
The time needed for the relief of symptoms caused by stress cannot be determined exactly because it depends on many internal and external factors (e.g. physiological processes of the seedlings, environmental conditions, care conditions, etc.). More information on the growth and care of the seedlings as well as the tomato crop can be found in our articles in the Armuro Community: Tomatoes, information about crop management and Tomatoes, treatments against pests and diseases.