My wife's 2002 Mercury Sable we purchased a year ago has red coolant in it. I found the owner's manual online and it recommends Ford Green or Ford Yellow. Will this coolant damage her vehicle? Can it just slowly be replaced or does it need to be flushed? She also seems to have a coolant leak we cannot find.
The antifreeze if actually red (as opposed to orange/red) is usually heavy duty and is specified for use in big trucks. This antifreeze should not hurt your vehicle.
If it looks more orangish, it is probably a Dex-Cool derivative. This should not be mixed, but will not cause issues if it is. Dex-Cool is an Organic Acid Technology (OAT). It has an extended life, but if mixed with green or yellow coolant, reduces its longevity, which is usually 2yr or 30k miles.
Propylene Glycol is the "green" stuff. It has replaced Ethylene Glycol as it is less toxic.
Hybrid Organic Acid Technology is usually yellow.
It appears that any of these technologies can be mixed without problems, though personally I would completely drain/flush your coolant system, then replace with whatever brand/type you are going to use.
Here is a chart of different coolants, their colors, and what technology they are considered.