You'll see them at various stages of molt at this time of year. I'm not positive, but the ones still brown now may have been birds born last year. The birds already in breeding plumage may be older...but I'm not all up on molt/age patterns in starlings.

BN

Don't know much about starlings but I always thought that the lighter colored ones are the juveniles and the darker ones are the adults...

Surya, I just read "After the first molt, juveniles have grayish brown plumage with lots of
white spots (see photo on left) on their head and breast, and they also
have a brownish/blackish bill - they almost look like a different kind
of bird. It is hard to tell sexes apart." here: http://www.sialis.org/starlingbio.htm.  Whwo, I'm thinking you might be correct.

When I lived in Mississippi, the seasonal color difference was quite dramatic. But never have I seen both colors at the same time!

Probably spotted ones took better care of their feathers :)

"Starlings turn from spotted and white to glossy and dark each year without shedding their feathers. The new feathers they grow in fall have bold white tips – that’s what gives them their spots. By spring, these tips have worn away, and the rest of the feather is dark and iridescent brown. It’s an unusual changing act that scientists term “wear molt.”