They are probably cold.

The cold may be tying up any available phosphorus. Adding more won't necessarily help.

if you can warm the soil that could help.

One way to do so is to encircle the plant and some of the soil around it with some stiff tar paper or roofing felt. The dark color will drawn in heat.

If you think the soil is warm enouh you could try adding some bloom fertilizer. Very few organic fertilizers have enough P to help.

One way to tell if the soil is warm enough. Sit down on it for awhile. If it feels cold on your bum, its probably still too cool for tomato roots.

Found this post last year which discusses the purple stems, leaves etc...But it is picture intensive, I have dial up so took forever to load..good info there..

Happy Gardening

Here is a link that might be useful: Purple stems on seedlings