What did you see? Originally, the center of the image appeared white, and the surrounding area was black. However, when the surrounding area was made pure white, the center appeared as an off white color (if you did not notice this effect, you probably have something in your view that is brighter than the center of the image). The important point here is that the light area in the center of the figure did not change. In fact, the light area in the center of the image is not pure white. The Info palette in Figure 2 shows the color values for the center of the image in Figure 1. It can easily be seen that the center of the image is not as bright as it could be (the brightest that it could be is 255, 255, 255), and the center has a blue cast.
As simple as this exercise is, it demonstrates two very important points for photographers.
This has important ramifications for photographic prints. If the white point in a print is not neutral, the eye of the viewer will change the non-neutral white point to pure white and will shift all of the other colors in the image in reference to that white point. In other words, having a white point that is not neutral will cause the colors in an image to shift in the eye of a viewer -- this is definitely not a good thing.
So, here is the bottom line: Unless you have very specific reasons otherwise, you should try to make the white point in images neutral.
Now, there may be cases when you do not want a neutral white point. For instance, there may not be any pure whites in the scene or objects that were photographed. This is demonstrated in Figure 3. The scene where this image was taken had no neutral whites. Attempting to create a neutral white point would cause a major color shift in the image.
At this point, you may be wondering about the black point. However, the black point does not have the same effect on colors (at least not nearly as strong of an effect) that the white point does. Thus, a non-neutral black point is not likely to cause a significant color shift in the eye of a viewer.