Hi! I read Ilndmon's question about the "yellow" spots on the lower leaves of her tomato seedlings but the photos did not reflect my specific problem. I have 4 tomato seedlings, now much heartier than a week ago and about twice as large as I'm in zone 7 (coastal NC). They're in roomy pots to allow for growth, and 2 are fine but 2 have orange, even rusty looking spots on the lower leaves, which are a deep mossy green. The spots are not yellow, though I realize color is subjective. Either way, there is no brown around the spots as there is on her photos. Unfortunately, my phone takes crummy photos and my camera's down right now so I have no pix, sorry.
Different kind of spot on tomato seedlings
Your extension agent may be able to help. I also listed some links that were helpful for me. I hope this helps.
North Carolina Cooperative Extension NCSU
New Hanover County
6206 Oleander Dr
Wilmington, NC 28403-3822
(910) 798-7660 Phone
(910) 798-7678 Fax