Why does this kind of screen chenge color at different angles?

It's a bunch of cheap LCD monitors stringed together, you can see their edges. They tend to have very poor viewing angles, the image starts shifting color hue as soon as you move off-center and it becomes impossible to see as you approach 180° (seen in your pic at the far end). But it doesn't matter in this case because the writing is very large and most people will look at them head on. It's actually a very good use of cheap LCD panels.