as one of the answers already mentioned reference to «MED.B.075 Colour vision» paragraph of 1178/2011, I won't paste it here again. That is the important piece of legal poetry that makes decision whether you fly at night or not. Or whether you get class 1 medical or not.
Important part is, there is no «color blind» terminology in EASA legislation today. There is «color safe» instead, as it was again mentioned before here in this topic. If you fail Ishihara test (like I did), you either accept your fate by confessing that you are not color safe, or you demand more sophisticated research. I was told by medical practitioner that Ishihara test does not show how severe your red or green deficiency is, it just exposes that such condition exists.
CAD test, being on top of «being very intelligent, super-duper piece of software that is impossible to fool because it is so clever and so many man-years of scientific effort has been put into it»-kind of hype, on the other hand, can show severity of condition somewhat reliably (as they say). There is a certain threshold, degree of deficiency that is acceptable for class 1 medical or class 2 night rating. I failed to resolve 4 Ishihara plates and failed CAD test miserably. Alas, nighttime is sleep time for me :) On the other hand, I have heard of folks that failed Ishihara but passed anomaloscope (see below) and won the class 1 medical prize.
Besides CAD test, you can choose to do a latern test or anomaloscope, which are older tests but still acceptable by authorities AFAIK. I don't remember by heart, but some exotic scrutiny exists in addition to these three. Being successful with any of the above, you get an override of your failed Ishihara test with positive outcome.
All said above applies to EASA jurisdiction.
Some countries have totally different set of rules. US has SODA that can potentially make your life much easier, granted that you actually resolve the colors that are actually needed at nighttime (e.g. you actually see that runway threshold is green and those beacons over there are red etc). Australia is very liberal in this regard and allows color blind people do more in aviation than any other state on this planet.