The universal color for a healthy vegetable plant leaf is some shade of green. For tomatoes it might be dark green, for kale it might be a deep blue green. There are some exceptions to this obviously with things like red cabbage, purple lettuce, etc. but what causes an otherwise green leaf to turn a different color? A reaction to an external environment that is not favorable to growth and plant health. A.k.a - stress. There are different kinds of stress, but generally it is going to be light stress, nutrient stress, pest stress, temperature stress, and water stress.
Low Nitrogen - Nitrogen is a key growth regulator. A plant that has lots of nitrogen will be deep green, but a plant lacking nitrogen will turn yellow, and ultimately new growth will be stunted. This is a complex process, but if you are interested in learning more, see our blog post on the role of Nitrogen in the garden
Lack of sunlight - Plants generate energy from the sun, this energy is created in the leaves. The sugars the plant creates through photosynthesis will fuel more new growth. Plants with dense foliage can sometimes cause their own leaves to suffer. Plants can also suffer if there is not enough sunlight to begin with. This suffering can be seen by yellow leaves, weak growth and brittle stems. The leaves are like solar panels, but they do not work collectively, rather independently! Which is wild that they are part of a bigger plant, but essentially competing for light against other leaves on the same plant. If your plant becomes too dense, consider pruning the inner leaves out to allow access to more sunlight. If your garden is located in a shady spot, consider moving it to a location with around 6-9 hours of sun.
Overwatering - when a plant is overwatered, the roots become susceptible to root rot. This reduces the plant’s ability to uptake nutrients and water. A plant can also get overwatered if they are planted in soil that doesn’t drain freely such as in clay soil.
Phosphorus deficiency - Generally used in the formation of flowers and roots, a phosphorus deficiency will be seen in purple stems, and then ultimately purple leaves. The purple can also take on a reddish hue in plants like corn, but it is still a shade of purple.
Magnesium deficiency - With anything, there will always be a common and an uncommon cause. Magnesium is uncommon but worth mentioning if you use bagged soil, or soil high in peat moss which can be void of many of the trace minerals that cause these deficiencies. Try using epsom salts once every few weeks when you water to reduce the chances of this.
Brown is the final stage of life for a leaf. Brown is the process of the leaf cells dying, then drying. A brown leaf is a dead leaf it can die because of age, or a prolonged exposure to stress for too long. Certain parts of the leaf may turn brown while other parts of the leaf stay green. This is known as localized die-back and is a protection against disease and pests. If a bug takes a bite out of a leaf, it will turn brown and dry only back far enough to prevent further infection to the leaf.
Frost damage - Black leaves are a symptom of frost damage. This is due to the cell wall rupturing from the expansion of water freezing. A black leaf will not grow back and will die.
Green veins & Yellow leaf
Iron deficiency - If your leaves are turning yellow but only in the veins, this is known as chlorosis. It is caused from a lack of iron. A good solution to this is using either blood meal or chelated iron. Both will fix the problem.
White / silver
Sunburn or sunscald - when parts of leaves or whole leaves turn white and thin, or even silvery, this is a symptom of sunburn. The cause is generally transplanting plants outside without hardening them off. Sunburn can also be caused from watering during periods of direct sun during the hottest part of the day. A sunburned leaf will not recover and ultimately will be dropped from the plant.