The holes that aren't used are for features you car doesn't have. Such as different security system options, transmission options, etc. JDM cars have more stuff in the gauge.
If you pull the bulb out of the base, it should have a number on the side. I can't remember what it is off hand, but I get them at local parts stores. They're just a common bulb.
I never put the green bulb condoms back on, often when a bulb burns out, they don't come off without breaking. Because as it burns out it gets really hot and melts the rubber.
There's a green film on the back of the gauges, so it'll still be green, just a tad lighter.
If you want to change the color, you have to pull the gauges out of the cluster and sand the green film off.