Seedlings purple underneath

[su_ju]

I have managed to keep my tomato seedlings from getting long and stringy (leggy) this time, but they are turning purple on the bottom of the leaves. Now, in the little reading I've done, this seems to be either too cold, or not enough phosphorus. Is this correct? Would simply putting them in a warmer spot (with light) remedy this, or do I need to give them some other help. As I am trying to keep this organic (and cheap!) anything I add should be organic. I planted them in a seed starter that is compost, vermicompost, and coconut coir. It says .65% nitrogen, .25% available phosphate, and .15% soluble potash if that makes any difference.
Thanks to all who make this an awesome site!

[BrianSkilton]

maybe the color-temp / spectrum your giving them? I use 6500k color temp. There was another topic similar to this one, maybe try searching the board to see if you can find it. If I find it I'll post it.

[hendi_alex]

I wouldn't worry too much about that purple pigment. Perhaps set an incandescent light very close will give some additional yellow/red part of the spectrum and will warm the plants a bit more as well.

[su_ju]

I have had the seedlings under a simple compact fluorescent bulb. It's output is only 800 lumens, 13 watts. I will try the incandescent today too. Fortunately, the sun is out today and I think it's supposed to stay out for a few days, so they will sit in a window to soak it up! I also put a heater in the room the plants are in, so hopefully, my peppers will make their appearance too.

[hendi_alex]

Peppers do not like to germinate in cool soil and will often just wait until the soil temperature warms or will sometime rot in the cool, damp soil. If you have some way to get some bottom heat on the pepper containers, they will likely germinate in just a few days.