The single biggest reason a lawn isn't green in the summer is that people don't know how to water. Put fertilizer on a lawn that is not properly watered and you risk damaging the lawn. Water the lawn every 2-3 days to a depth of 1 inch. Put a can or glass on it when watering to see how long it takes for 1 inch the first time. It's best to water deeply several times/week rather than water everyday. Everyday light watering will keep the grass roots near the top of the soil and they'll go dormant or die in the summer. Water heavily and the roots will go deeper into the ground. Light watering also encourages weeds. In about 2 weeks your lawn should become greener. If it doesn't fertilize it and then continue watering heavily every other day or so.
You can also use Epson Salt according to the directions on the package. You can buy it at any pharmacy or some garden stores.
Also, you might consider an iron supplement. It doesn't actually "feed" the grass like fertilizer. Also, it doesn't green the plants as long as nitrogen, which is the greening agent in fertilizer, and must be reapplied more often. Happily, iron doesn't make the grass grow faster and thus mowing chores are not aggravated. Also, I don't think you can damage the plants like you can if you put too much on like you can do with fertilizer. Most people just use fertilizer, because it has other effects, such as a thicker lawn.
There has been some concern regarding fertilizers and other chemicals used on lawns, such as phosphate runoff and runoff of herbicides and insecticides. I don't think this issue exists with iron supplements.
Nitrogen is what makes your yard green. Nitrogen also makes it grow extremely fast, so be prepared to mow a lot more often. I used all kinds of fertilizers over the years and kept looking at my neighbor's, which had the greenist lawn. I realized they were using Scott's. I started, and now mine stays green all year.
To keep your lawn looking great all summer, just follow a few easy lawn matienance tips:1)Feed your lawn regurally: If you feed your lawn with a slow release fertilizer (such as Scotts Turf Builder...
It can. Lime is used to increase the pH level in the soil. If you pH level is below 5.5 then adding lime will improve the soil conditions and it would therefore make the grass greener.
Your weeds are 'eating' all the nutrients meant for your grass so they are healthy and your lawn isn't. I suggest pulling them out and making sure ALL the roots are removed. :)