What is that sprayed Green Liquid with grass seed for lawns?

I've seen ads on tv for this stuff. It's a green liquid which contains grass seed, and you attach it to a garden hose and spray it on the lawn to grow grass.
At first I thought it was just another gimmic to make money (and it's costly), but now I noticed the county sprayed some of it where the earth was moved for some construction.
Apparently if the county is using it, it must have some benefits over plain grass seed.
Obviously it contains grass seed, but what is the base material that turns the soil green? Does it contain some fertilizer or some other materials that assist in growth of the seeds? Are there other benefits?
Lastly, am I corect to assume that the water is added to a dry powder? I'd guess that if the substance was sold as a liquid, the seeds would sprout in the container.
I have NOT seen this sold in stores, but I'm not sure what it's called either.
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snipped-for-privacy@unlisted.moo explained :

Instant Lawn?
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snipped-for-privacy@unlisted.moo says...

Google "hydromulch"
--
RonNNN

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On Wednesday, July 27, 2016 at 5:01:48 AM UTC-4, snipped-for-privacy@unlisted.moo wrote:

The product the pros use is grass seed, fertilizer and hydraulic mulch. The DIY thing is probably the same. What you see on the ground is the hydraulic mulch that has dye in it. The hydraulic mulch is a cellulose product, kind of like paper mache, that holds water, which helps get the seed started, prevents erosion, etc.
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On Wednesday, July 27, 2016 at 10:33:48 AM UTC-4, trader_4 wrote:

Forgot to add that if you want to use that kind of product, you can buy the hydraulic mulch at places that supply landscapers. It comes in bags the size of fertilizer bags. You can then use your own seed and fertilizer. For any larger areas it's going to cost a lot less than the advertised, pre-mixed stuff. This spray it out is just the latest extension of the push to get consumers to buy expensive packaged products. HD and similar have been selling pre-mixed hydraulic mulch, fertilizer and seed for a long time, the kind you just spread out, then water. People see a big package and think they are getting a lot of something compared to a nearby bag of pure seed. Actually there isn't very much seed in there. A similar size bag of seed will probably do 10x the area.
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