Or, if they're not painted or have anything over the bulbs (which I doubt - the Germans simply didn't do anything like that), then the Tach has "Euro" filters fitted over the bulbs within the tach assembly. This is a little piece of colored (green, in your case) plastic film surrounding the bulb once it pokes it's way up inside the housing.
Only way to change them is to take the back cover off (not an easy thing to do, but still do-able) and either remove the film and replace it with a different color (usually just green and clear) or just leave it off and it'll probably match the other two gauges.
To separate the case from the front glass the bezel has to be gently pried away from the "can" until it releases the front bezel and glass, then remove the screws on the back of the can and slide it off of the "guts".
Now that I read all this, it might be easier to call whomever you bought it from and just get them to exchange the right one for what you've got, eh?