My yard was hydro-seeded a little over 2 years ago (spring 2012) Kentucky BG....
My yard was hydro-seeded a little over 2 years ago (spring 2012) Kentucky BG. Yard is 90% sun, as tree are so small, they do not provide any significant shade to the turf. This year a new grass is spreading at am alarming pace. It is lighter green than the surrounding BG and is growing in dense patches
Wright County Minnesota
It sounds like rough bluegrass, Poa trivialis. It is difficult to remove and you may need to use glyphosate (Round-up is one brand) to kill it. It will also kill your desirable grass. Less watering might help, but we can't control how much rain we get.
Here is some information on it and its control. Note that this link is meant for commercial use and it mentions some chemical controls that are not available to the homeowner.
Will it turn darker green as it matures? Or will it always be a noticeably lighter green shade?
Sorry, but it is a different kind of grass and will never match. There are lots of kinds of bluegrass (some people seed their lawn with rough bluegrass) and some might blend better, but if you are seeing a noticeable difference now, it will continue.
I have a pretty good infestation of the Rough BG (30%) and want to avoid killing everything with Roundup. Do you this this might be a reasonable first step in controlling/killing it? Cut back on water Treat with 2-4D Pulling plants on leading edge. They pull out so easily and do not appear to have shoots , like quack grass.
2,4-D will not kill it. You could spray the areas you don't like with glyphosate and replant with your preferred bluegrass, but you would need to know what kind that is. There are over 200 varieties of Kentucky Bluegrass. If you look at the back of the bags of Kentucky Bluegrass seed, you will see the varieties that are in the bag. Many contain Poa trivialis, which is what you want to get rid of.
Here is a link on how to renovate your lawn, which you could use if you decide to kill and replant or re-sod. http://www.extension.umn.edu/garden/yard-garden/lawns/lawn-renovation/