Answer: If you water and fertilize regularly, the only reasons I can think of that might affect the color are 1) you have several different types of grass in your lawn and as the grasses colonize, each patch is a slightly different texture and color, or 2) the drainage is a little different throughout the lawn area and the lower spots are collecting and holding more water and fertilizer - hence the darker green color.
Although most lawns start out as a single grass type, within a period of 3-4 years native grasses generally begin to sprout up. Birds and wind are responsible for transporting the seeds to your lawn. You can dig these other grasses out and reseed if you'd rather have a single grass type in your lawn (and thus a more even color).
Best wishes with your lawn!
Q&A Library Searching Tips