Hello Tom,
The most used static grass is either 6 or 12 mm and you can get it in differing colours.
My method.
Take some of the grass from each of your shades and sizes and mix them all together untill you get close to the shade you are looking for.
I would also get hold of some 'moss' and mix this in with some fine garden dirt, or if you do not have a garden, sweep the gutter for a few yards/meters and collet and seive that through a tea strainer. And while collecting it, keep muttering "damn dogs/squirrels/cats" because you will get strange looks from the neighbours....
. As a quickie fix, for moss you can use the dust from your grass as a starter.
Put some white glue/pva onto a small area, stick a pin into it and then attach the clip, pour some grass into the dispenser and turn it on as per the instructions.
NOW! If by chance it does not work as advertised, then put it into the tools I bought but will never use section.
Method two (it works)
Put some pva onto your base, do not go mad about 50x50mm take a handfull of your grass and let it fall onto the area covered, then with what is left in your hand put that on the base and gently tamp it down. Once that is done turn the base upside down and allow the lose to drop onto a sheet of paper, you can use the grass again. Then using a tea spoon load it with 'moss' and sprinkle over what you have just done and repeat the upside down onto another sheet of paper, then leave to dry while you do other areas.
After about half an hour of tamping and turning get the vacuum out and the round nozzle that is attached to the brush head will be used next. Take a fine cloth (handkerchief), wrap this over the end of the nozzle and hold it tight otherwise it will go up the vacuum.
Then play the nozzle over the new laid grass at a height of about 10 mm. this will suck up the loose grass/moss and after a minute turn off the vacuum and remove the cloth and then empty what has been collected onto your paper. Experience will tell you how close you can get to the base, and you will also see the grass start to stand up and as you move the nozzle so you can add wind movement to the grass.
The pic below might help you out, in the mix are 'leaves' and three types and sizes of grass as well as moss.
Cheers,
Mike.