The Holmes Wright test, you sit 5 metres away from the box. It shows you two lights one above the other. They can be White, Green or Red. The lights are VERY small ( just over 1mm I think ) and they show them for 3 seconds. They do change the Hue (taking colour out is the best way for me to describe it) and it is generally the white and green we mix up. There is nothing in real life that you could compare, but try to think of looking at traffic lights at a few miles and you won't be far off. All the very best to you mate.