If you notice that your car's antifreeze is coming out of the overflow reservoir but the engine isn't overheating, it could be a sign of a serious problem. The most likely cause is a faulty head gasket. This seals the engine block and cylinder head so that coolant doesn't leak into the combustion chamber. If the head gasket fails, it can allow coolant to enter the cylinder and mix with the oil. This will cause the engine to overheat and may damage other engine parts as well.
If you think you might have a blown head gasket, it's important to take your car to a mechanic right away. They can perform a pressure test on the cooling system to check for leaks. Once they identify where the leak is coming from, they can make repairs and get your car back in working order.