There are a number of issues that need to be examined.
1. Is colour vision really needed and relevant in aviation?
2. Does the CASA Medical Section have the ability to move with the times and apply modern thinking?
As a young pilot we had to perform such tasks and tests that seemed relevant, but at the best they were obscure. The Nav light test was one that I can think of, surely TCAS has taken care of that? I am joking, but never have I had to look at another aircraft at night and decide is it coming toward me or going away. Modern airliners have a number of aids to navigation and safety enhancements that make piloting skills more about management of systems than raw talent. I think colour detection falls into this category. Besides in a two crew environment any deviation from laid down parameters will be brought to the attention of the other pilot. I believe if you did a risk analysis of the threats that this colour deficiency poses and the mitigators would make the risks in the low to medium range.
CASA's Medical Section has over the years shown it's decisions and attitude has be driven by who is leading the section. At the moment it is very risk averse and would rather give decisions that ground pilots and justify it in the interest of safety. The problem with this is that it interferes with peoples lives and livelihoods for arbitrary reasons. I have known a number of people who have lost there medical due to injury and it has taken a extraordinary amount of time for them to get it back, even when the weight of professional opinion has said that they were fine. I believe colour defects fall into this category as the weight of evidence suggests that it has no relevance in aviation.
Finally lets talk about ICAO and it's guidelines. Although ICAO is an entity in it's own right it is made up of member states, these states contribute to policy and guidelines. ICAO does not get it right all the time and if they are anything like any government department in Australia or even the UN they get it wrong, the guidelines are out dated or there is some vested interest at play. In the case of colour defectiveness the guidelines are out of date and have no relevance in a modern aviation environment. Australia used to lead the world in aviation thinking, but, in recent times we have stepped away from that role. This is one area where we as an aviation community can show leadership and make change for the better.