Everybody (okay, nearly everybody) wants a nice green lawn. How do you do it? It's not so hard really. First start by watering properly. As glib as that may sound, it truly is that important. Without proper watering, nothing else you do to the grass will make it turn green. Read our Watering FAQ for all the details.
Next, apply a little fertilizer over the course of the summer. You can read the Fertilization FAQ for specifics, but without some nutrients consistently added through-out the growing season, the grass will fade and become less green than it can be.
Since you are watering and fertilizing, the grass is going to grow. If the lawn is cut improperly, it will not be as green as it can be. Simply put, mow high and let it lie. You'll the 'cutting details' in our Mowing FAQ.
Put these three practices together in the right combination and a green lawn is actually quite easy. But remember, once it gets green, there is a maximum level it will be. Once it's looking good, enjoy it and don't keep trying to make it greener still. You'll be putting the lawn into a stress condition, which will eventually cause major problems.