Question
The color of my lawn is patchy with soft green and dark green sections.  I'm guessing this is from using different types of lawn seeds which produce various shades of green.  My question is can I use some type of fertilizer which will give the lawn a uniform color?  I live in northern Illinois  

Answer
I wish I could give you the answer you want to hear, Bill.  There's no such fertilizer -- and if one says it does, don't waste your money.  Grass color is genetically determined.  It's in the DNA, not the fertilizer.

If you're sure the variations in color are from different seeds, and not different sun exposures for the same seed (full sun here, shade under a tree there, more sun, shadow of a building, dark under a hedge), and you don't know which one you have, consider overseeding this fall.

Overseeding is a regular practice among Lawn lovers.  Every few years, they revive their Lawn with new seed.  Usually the same variety, or an improved one if there's one available.  Grass gets tired; this is not a waste of time and money.

To overseed, pick out your favorite Northern Illinois Grass on the internet -- they usually sell the best stuff there --  and put it in the freezer.  (This accelerates germination in some varieties.) Around 8 to 10 weeks before your first frost date, rent a seed slitter or you can scratch the seed into the soil.  Water daily until the Grass is underway.  As you mow, the new vigorous Grass will overtake the old Grass.

Make sure you mow at the right height for your Grass.  And of course you should not use any chemicals.