The multicrew environment is nothing to do with medicals. I was hit by the same thing myself. It's no loss other than to the ego....
I would suspect that an initial medical issued by the UK CAA at Gatwick in accordance with Part Med is not going to have been issued by accident. Especially considering the fact that he went down there specifically to seek a means of compliance for getting a Part FCL medical while it is clearly known that he is colour blind.......