There's colour blind, and there's colour blind. I have a mild deficiency but that hasn't stopped me from being an ATC in three different countries (Oz, UAE and Hong Kong) over the last 38 years. One is trained from almost day one (as a child) to put a certain name to what one is seeing, it may differ slightly from what a normally sighted person is seeing, but so what. Red is red is red as long as all in the group have been told that "that is red". I have little hope of passing the more complex "Ishihara" colour plates, but have no trouble with lantern tests. Most aviation authorities will go to the lantern test as a back up if you can't pass the colour deficiency plates.