It could have been almost anything. The two main ingredients that help a lawn green up quickly are nitrogen and/or iron. Nitrogen is the more likely culprit unless your soil has an iron deficiency or is alkaline.

If you're applying chemical fertilizers, you want to apply approximately 1 lb of actual N per 1000 sq ft. For example, if you have 10000 sq ft of lawn and you're using 46-0-0 fertilizer, you'd want to use a little more than 20 lbs of fertilizer. Divide 10000 by 1000, then divide the result (10) by .46 to get 21.739 . . . I rounded to 20. If you're using 21-0-0, you'd want almost 50 lbs (10/.21 = 47.619).

Thanks, if I have 1/4 acre lot, how much N should I put down? Will N hurt my lawn this time of year (mid-Dec) in VA or should I wait until spring or early next fall like they did?

What kind of turfgrass do you have? If it is a warm season grass like zoysia or bermuda, you would have to overseed with ryegrass to make it green in the winter. If it is a cool season grass such as fescue or bluegrass, then go ahead and put down the nitrogen and it will extend the green color. My suggestion would be either 46-0-0 urea or 21-0-0 ammon sulfate, but any fertilizer with a decent amount of nitrogen (the first number on the bag) that is in a quick release form (no sulfur coat) will work fine. With a 46-0-0, use about half the bag or so for a quarter acre lot. With 21-0-0, use the whole bag. Depending on your soil temperatures, it may not be able to green up your yard until the spring. Ideally, you would want to do this application just before Thanksgiving in your area, IMHO. But first, know whether you have a warm season or cool season turf type. Good luck.

"if I have 1/4 acre lot, how much N should I put down?"

Depends on the ratio of N in the product you are dropping.

FYI - 1/4 acre is 10,890 sq. ft. - just a little more than the given example above.

DenverDude makes a good point about the type of grass that you have. In Zone 7, you could have either a cool season or a warm season grass.

As Gary pointed out, your lawn is pretty close to 10000 sq ft, so you could use the calculations I gave earlier as a guide.