...GET IT OUT OF THERE NOW!!!!!!! TAKE IT SOMEWHERE AND HAVE THEM COMPLETELY FLUSH IT!!! If G12 (OAT inhibitor) coolant is mixed with conventional coolant that uses silicates as an inhibitor are mixed, they turn acidic and start eating your engine *QUICKLY*. They also turn into a gummy substance and you get chunks flowing around and plugging things up. Flush it COMPLETELY, blow it out with air and flush it again.
The VW TSB that talks about retrofitting cars that came with blue G11 (very similar to green Prestone minus the phosphate inhibitor) says to do a thorough flush and blow out with air, you don't want them mixing.
If *any* fluid is a different color, you need to be on alert that it probably isn't compatible. Even same color fluids aren't compatible a lot of the time (take dex-cool and damiler-chrysler hybrid-OAT coolants, they're both orange but they're not the same)...
Yes, G-12 is proprietary, but many, if not most, other cars are coming with a proprietary coolant. Ford is the only one that's using the old green stuff in almost everything. I think the new Cougar is the only thing that they used OAT in for a while... Japanese coolant is low silicate, high borate. Very few things mix...
.. if your Passat is 10 years old, it must have originally come with G11 blue coolant. I'm assuming someone did the proper flush and change!?!? VW started factory filling with G12 in mid 1997 model year, right around the same time as they change to the single row upper timing chain....