It should be called purple-stemming. The stem goes purple first. If you don't correct this problem in time, the undersides of the leaves turn purple. If you don't correct that problem in time, the whole plant turns purple. Like this.
Notice as well how the little leaves are pulling upward, instead of branching outward. That's another sign of stress.
Tomatoes need a lot of soil nutrients. If you grow your seedlings in a peat mix, with no real soil and no fertilizers, it will get red-stem.
The solution is to give the tomato a quick dose of liquid fertilizer. Then replant it in rich soil. In a few days, the purple will give way to green.
Small seedlings that get a bit of red-stemming survive just fine. But the longer they're red-stemmed, the lower their chances of producing a good crop of tomatoes.
The stress can be caused by cold temperatures. But it's often a sign of disease.
This tomato has some kind of blight. Note the spots on the leaves.
If your tomato starts doing the twist, pay close attention. It's trying to communicate that something's wrong.
Peppers get red-stemming too. They take longer to show signs of poor soil than tomatoes do. If your pepper leaves turn purple, that's a sign that it's been grown in soil that's nutrient-poor.
Avoid giving peppers a big dose of fertilizer until they're flowering and fruiting. Otherwise you'll promote leaf growth and discourage fruit growth. Instead, repot it in real soil.
Seedlings grown indoors or in greenhouses aren't used to the sun's harsh UV rays. Believe it or not, the seedlings will get sunburn.
Leaf sunburn looks like silvery patches, which soon turn crisp. The plant suffers because of the loss of leaves. It will recover, but your harvest will be delayed.
Always harden off seedlings before planting outside. The general pattern is a 1/2 hour of direct sunlight per day for 2 days, then 1 hour for 2 days, then 1 1/2 hours for 2 days, etc., until you reach a full day.
It takes about two weeks to harden off a seedling.