Telboy,
Whilst I sympathize with your predicament, you do (No pun intended) rather see things in black and white.
A tower signal test has no real relevance whatsoever to the subtlety of colours that make up the reality of commercial aviation operations. For example most flight displays these days are CRTs and LCD displays. Failures of colour guns, tubes or generators can cause problems with normal colour vision. Have you considered how those problems may change for people with colour vision deficiencies ? The same can be said for airport lighting systems, where violations can already be problematic without adding another deficiency to the equation.
It might well be that other countries licensing and medical authorities take a viewpoint based on their own perception of risk in those countries. There are already many different standards of flight operation that some countries find acceptable, that others do not.
In the USA or Australia it might well be that airline screening negates the need for stricter state medical standards in any one particular regard, such that the likelyhood of a problem occuring because of a deficiency is considered "sufficiently" remote to be a problem in that jurisdiction. However that does not in itself mean that such a standard should be considered as prudent or acceptable everywhere else.
You make a lot of assumptions about flying N reg 747's around the world, because of course you don't. If you are one day in the position of doing so, then bilateral and international agreements would normally afford you the courtesy of continuing to operate to a different countries licensing requirements whilst operating abroad. Indeed that happens with almost every international flight that occurs. However this does not mean that a country or in the case of the JAA ( a supranational collective of countries) should necessarily adopt another countries standards as its own. This courtesy does not make you either unsafe or indeed safe in itself. In all probability it is simply a perception of acceptable risk.
On your final point, the CAA are clearly not "negligent" in allowing FAA pilots to operate in this jurisdiction for the reasons I have already given. I doubt there are many foreign pilots operating within the jurisdiction that are not at a standard acceptable to the authority, and for the few that might be, the risk may be deemed acceptable. Are the CAA discriminating against pilots with a high CVD ? Yes that is their job. They are obligated to discriminate in a way that ensures compliance with the standards laid down at any given time. These standards apply to other health issues and not just visual accuity.
Finally, I do wish you well, and hope that you are able to find a way to overcome a difficulty that obstructs your career path. However suggesting that medical authorities are being either negligent or "stupid" in this regard, is being overly simplistic and failing to consider any other viewpoint than your own.