The colour blindness issue is a red herring.
Historically, barring some flying privileges to colour blind people had surfaced for fear that such people would be unable to distinguish the different flare signals sent by a tower in the event of a radio-communication failure or unable to distinguish the left and right position lights of a conflicting traffic. Most colour blind people can perfectly distinguish the 2 position lights but fail the Ishihara test.
The real issues that have a causal relationship with this accident are:
- was the pilot trained and current for flying in IMC at night ?
- why wasn't a commercial pilot at the controls of that plane, if it is confirmed that the actual conditions of the flight match private carriage operation as defined under FAR 91.501(b)(4).