Sometimes the leaves of the tomato seedlings we grow in pots gain a purplish shade on the bottom; the plants themselves (don't know if this is a coincidence) also seem to turn out stockier, shorter, and thicker in the end, and curl a bit.
Does the purple color mean anything? Is it a problem, and if so, are there cures?
I've heard of phosphorous deficiency or cooler temperatures, and cooler temperatures causing the phosphorous deficiency. It happens to my seedlings every year, and shortly after planting them in the ground it goes away, never has been a problem for me.
You've got to pay attention to any insects in the area and how moist the soil is to work out which of two possible reasons for the purple rain.
You've got this if the leaf veins are purple and overall sport a purplish tint. "Recognizing Tomato Problems - Colorado State University". Most likely if there are no insects around, it's early in the season and you're bathing the plant in cool temperatures or the soil itself is too wet. The bedding may also be passing its own phosphorous deficiency to the plant.
This can usually be fixed by putting the plant in a warmer area and letting that soil dry out a bit.
If you want to give it a boost, throw some bone meal around for that extra kick and help with the phosphorous.
Stunting, leaflets rolled upward with a purplish colour, especially along the veins and yellow looking leaves may indicate a diseased plant. This is also known as beet curly top virus (BCTV). If you notice some insects hanging around, and the plant isn't producing as much as it could be, that's another sign.
Infected leaves of some hosts, particularly tomatoes and peppers, become thickened and crisp or stiff, and roll upward as the petioles curve downward. The leaves turn yellow with purplish veins. [...] The occurrence of this disease in home gardens may be due, in part, to the presence of alternate hosts that leafhoppers prefer to feed on, as well as an increased likelihood of infected source plants in the area.
You can't spray this problem away and is slightly inverse to the conditions of phosphorus deficiency. You've got to keep this affected plant in the shade and away from the hot sun, where the insects love to spread their ills. Cage it from invading insects if you can until it's mature enough to stand on its own. Or, just get rid of the infected plants and start anew.
My first guess would be that you maybe overwatering. There are some insect/fungus reasons that they may be like this, but purplish leaves may be as simple as it still too cold and you need to back up on the watering.
I start my garden veggies like tomatoes, indoors and have noticed a purplish color under the leaves as well. It's definitely not bugs or a virus. The ONLY factors it could be in my case are ambient / temperature being too cold - @68 °F, or too much fertilizer.
I've been starting my veggies in my basement for over 10 seasons now, and just noticed the discoloration, which coincided with an application of fertilizer at @ 700ppm in strength with week old tomato plants. I usually add just tap water (@125ppm) and have never had a problem with purple leaves, but I grabbed the wrong container of water (fertilizer water @700ppm) and didn't notice it until the next day. I drained the water and soaked plants in tap water to purge any fertilizer out of the seedlings. They are all doing fine and continue to grow but still have the purple color on the bottom of the leaf with the veins dark purple as well.
I believe it was a result of a combination of cold water / ambient temp and the fertilizer locking out the seedling's ability to absorb phosphorus.
