I think that immediate direct sunlight is probably too much for them. I would shade them for a week or so and then gradually put them into the direct sun. I have some that just came up as well and I have under some trellis. The trellis lets just enough light through.

Daniel

If you planted the seeds outside, when they germinated, they'd rise into the sunlight and be quite healthy and vigorous. And many people start their seedlings in cold frames that have lots of light. FWIW, I've usually found that young seedlings are lighter-colored than the plant as it ages. Must be something in the leaves that interacts with light.

Are you putting the seedlings inside by a very bright window or outside? If by a window, I'd wonder if they're just too too hot. I start determinate tomatoes in containers outside in the bright sunlight in June to harvest them in the fall, and I've never had a problem with the seedlings turning yellow. So I don't know what the trouble is.

Sunlight through windows can easily burn plants mainly due to temperature build up in the tissue. This is especialy true within a foot of the glass. There is less air flow than would be found outside, and outside you would have the moisture of the ground mass acting as a heat sink. This is why you have to be more careful of plants on a patio too. The cement holds the heat.

Thanks for the responses. The temperatures here have been in the 60's and 70's during the day and I've been putting the seedlings outside immediately after they are visible above the soil surface.

I'm beginning to wonder if I should just move the entire seed tray outside during the day and put it on the warm dark spot at night.

If you're seeds are germinated inside, regardless of the amount of light they receive, they will need to be hardened off.