Question
We live near Chicago and installed sod last summer (mostly Kentucky Bluegrass but there are other varieties mixed in).  The sod seemed to root well and was quite green before going dormant for the winter.  Now that spring has sprung (we've been in the 60's and 70's for 2 weeks) our lawn is not greening up very quickly compared to other lawns in the neighborhood.  The grass is not entirely yellow but I am concerned that I need to do something.

Answer
Let's see what the Iowa State Plant Pathology Dept website has to say about your question, "Spring is Here, But Why is My Lawn Still Brown?"
(http://www.ipm.iastate.edu/ipm/hortnews/1996/5-3-1996/brown.html) - and ignore please their advice about using herbicides -- in my opinion, anyone who would do that must be crazy:

"If the lawn was started from sod, it may not have established in some spots and these spots winter killed."

You didn't exactly say that your lawn is yellow/brown, or that it is looking paler in patches.  But you did say it is "not entirely yellow".  How uniform is this yellow?  Because patches points to one problem, general piqued coloration would be another.  Which one is it?

Go over and see if you can pull out clumps easily.  Check the roots.  Then report back to Lawn Central.  Thanks for writing.  We'll talk.