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.
Curly top virus (CTV)
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.
Curly Top Virus - New Mexico State University
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.