In the US and australia , if you fail the Ishihara tests, there are a number of alternative tests, the main one is the Falant lantern test.
If you cannot pass these, you can do a SODA test(staement of demonstrated ability) which usually involves identifying lights from a control tower at various distances.
Then you can get a SODA waiver with your medical which exempts you from further tests but must be showed to a prospective employer.
You can get an ATPL with this though if you are European, it might be difficult getting a green card/work permit.