Creampuff, I agree with you in a cockpit that has lights that are either on or off. My understanding is that the problem is when a light changes colour depending on the state of a system, or when the colours of digits change on an EFIS display. The message is still there, but the time to detect it may change. Whether that actually matters or not depends on the system and the circumstance.
An analogy is traffic lights. The position of the light in the stack provides a redundant cue to those with CVD, and in most cases this is enough. There is good research though, that shows CVD drivers (and not all CVD is the same) are slower to detect the red and amber lights than non CVD. There is no difference in green detection, and it probably doesn't matter. So what, you may ask? Well it turns out that in particular circumstances, that 40-50% increase in detection time does matter, and CVD drivers are over-represented in rear-end collisions at traffic lights. Not a lot higher, and they are usually low speed, but the data is clear. Simple system, the light is on or off, and there is redundancy built into the system (standard position of the lights).