You can put the tray in a window for light, however that may result in too-tall, bending, spindly plants. The best route is to rig up an incandescent light source that can be hung within 2 inches of the tops of the sprouts and that can be raised as they grow. This prevents them from growing too tall, too fast which produces leggy, weak plant stems.

Make sure plants have light for 18 hours--no more, no less. You can get an inexpensive timer device that will turn your lights on and off.

Keep in mind that too much light can be as bad as too much water or too much fertilizer and the opposite is true with not enough light.

The bottom of the stem and leaf undersides of tomatoes are usually purple. However your seedlings are leggy and that means they are not getting enough light. If your grow light is a single bulb it is not much good and for the most part with gardening things they are mislabeled. The best light for growing is a shop light with reflectors to train the light downward and T8 daylight bulbs. They give you the full light spectrum. The lights should be no more than an inch above your plants but not touching the lights. Your seedlings are not lost however. Get the light right and transplant your tomatoes into pots. I use disposable red plastic glasses which have holes punched in the bottom. When you transplant sink the seedling into the soil up to the leaves. Roots will grow all along the stem. This is a quality that only tomatoes have but it makes for a good root system. I do it with all my tomatoes even if they aren't leggy

They are very leggy. Not nearly enough light or the light isn't close enough to them. When you transplant them into containers bury all that bare stem.

Dave

Thanks everyone for the replies. My lights say they are 7800K. Is that what I want? I am going to transplant them into containers soon, what kind of soil should I get?