Well, this is the only image I could find of a disco ball cake just for my own reference in color help.
http://www.partysquad.com.au/party-themes-disco.htm
I have used silver on white fondant and it takes a couple of coats. You can always die your fondant first. Usually that works better. It would be super cool if you could make the squares with silver leafing but I imagine that is expensive.
When I paint, I notice that it takes several coats to get it to the shine I want. Paint one coat and let it dry and paint another if you feel it's necessary.
You'll just need a little of the vodka or lemon extract , just enough to wet the brush and then dab into the powder with it to mix it up, load up the brush and paint a little on the plate and you'll see when it's the right painting consistency.
Good luck!
Cant guarantee the results but you've had one heck of a time painting a cake!!! 
You only need a couple drops of vodka or REAL lemon extract like 90 percent I think Put vodka in cap of jar with a little dust that way when you are done you wont waste whats left just let it dry in cap and screw back on jar. just get it wet enough to paint with. good luck.
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