In the US the airlines have to have a really good reason to stop you on medical grounds, issues like strong prescriptions and color vision are usually static and therefore if you have satisfied the FAA that you are safe then according to the Americans with Disabilities act who are an Airline to say different.
The only exception is where you hold a current first class medical but the condition is progressive, such as HIV, motor neuron disease, multiple sclerosis, and cancer that is not in remission etc. then the airlines are able to stop you on the grounds that they are going to invest a large sum of money in training you and they will be unable to recoup their investment before you lose your medical.
As for Europe, It is quite likely that a case like yours would stand a very good chance of success in an employment tribunal so it is unlikley that any airline could refuse you with a valid Class 1