You might be noticing your lawn is more blotchy and spotted now than normal. A lot of grass is producing seed right now. Different varieties of grass produce seed at different times of the year. As the grass produces seed it uses all of its energy on the seed so it loses its color. Not all varieties of grass are seeding right now, which is why there is a large contrast in color between the varieties that are seeding, and those that are not. This is what makes the lawn look spotty. This a natural process and no amount of fertilizer will darken up grass that is seeding. In fact, fertilizer may make it appear worse since the rest of the grass will turn darker and grow more and this creates more contrast in color.
Make sure your mower blades are sharp. Cutting through seed heads and thicker stalks of grass will dull your mower blades fast. Mow on a regular basis because the seed heads can make it very difficult to cut if you let them grow too high. Keep you mower blade set high, preferably 3 inches or higher.