Rather than guess, take a spade and cut a chunk of sod from one of the
yellowed spots and from one of the green spots (don't worry, you can replant
it easily).

Look for:
1) soil moisture well down into the root zone in both places, or just one?
2) do the root systems seem equally well developed in both places, or just one?
3) do you see insects, grubs, etc. in one sample?
4) do the roots look and smell the same?
5) are you dealing with similar soil types?
6) are there different amounts of thatch (dead material just above the soil)
7) do the grass blades look similar or different when you compare the two
sods? (same width, same basic structure?)

I'm guessing that you're watering too little, too often. About an hour after
you water, the soil should be damp about 4-6" down from the surface, with no
puddling or ponding on the surface. Deep, relatively infrequent waterings
encourage deep root growth -- 3/4 to 1" of water is generally recommended
at a time in Houston, then watch for signs of wilting before watering again.

My other guess is that the yellowing you're seeing is due to something
wrong with the root system -- a fungal disease, grubs or other insects,
or something odd in the soil (thin soil over rocks, perhaps?)
Watering help:
http://aggie-horticulture.tamu.edu/c...ter/lawns.html

I believe you said you were in Houston. You might also want to submit
samples to your county extension service.

Kay