I have personally taken the Nagel twice at two different places using two different devices. Both are as said matching the top and bottom semi-circles.
The first device was like a microscope on its own. There are two knobs on each side to twist to get the colours to match. I found it VERY difficult to operate. The second was a computer controled scope. The computer gives you choices and you have an appropriate button to press. After the test the operator set the computer to show me what a "colour normal" would see. To ME it was a distinct red top and green bottom, but this is MY view of it - others will see it differently.
Hope this helps, but it is a very hard test.
Good luck for Tuesday and all the best for your future flying.