What distance can humans still distinguish between different colors?
There are a couple of areas for you to explore further. One is called “small-area tritanopia.” That refers to the fact that the central area of our fovea has no blue-sensitive cones and our vision for small objects (less than 0.25 degree or so) becomes tritanopic (yellow-blue) color blind. The second are the “chromatic contrast sensitivity functions.” These provide data, like those you cited for acuity. Unfortunately the results depend very much on the definition of chromatic contrast and the viewing conditions. As a very rough estimate, you could assume that the ability to discriminate colors has about half the acuity of our ability to see the objects. So, roughly speaking, if you can just see the 2.5m object at 1700m, it would probably have to be 5m in size to reliably distinguish colors.
