Is my head gasket leaking?

So what do i ask the mechanic,and how much you think it would cost.

Owner, Ask for the Block check procedure with the test fluid ( which changes color if the system fails ) That should not cost more than their rate for one hour of Labor.

So do you tink the color change of the coolant is the oil mixing And its causing engin to overheat

Owner, ( This is not a diagnosis of your concern,that requires an on-site analysis ) ) It not the color of the coolant that causes the overheat, it is the combustion gasses pushing the coolant out of the cylinder head & stopping the circulation of the coolant which causes the overheating condition. Also, other possibilities are: Restricted ac condenser, restricted radiator, cooling fan malfunction, faulty thermostat, loose water pump impeller, failed harmonic balancer, cracked cylinder head or engine block issue.

Will that cause the car not to start when its hot and smell like gas

Owner: Faulty fuel pressure regulator,faulty engine coolant temp sensor, canister purge or canister vent solenoid, map sensor, mass air flow sensor, battery connection, battery, starter, or alternator issue, fuel leak, PCM., That is why there is no substitute for A Trained, Experienced Professional Technician with the correct testing equipment and procedures, which is the best investment by a consumer toward the repair of their vehicle.