The problem is that it is a projection, thus, at the bottom point there is not only black R0 G0 B0, but blue R0 G0 B255. At the upper point, there is not only white R255 G255 B255, but also yellow R255 G255 B0. However, if in the image there are no so many primary colors, it is often a third color, which correlates with two visible. R and G increases moving up, but B stays, so the color becomes yellow. When it is moving down, respectively, it becomes blue. If you turn on the button Proportional, the program changes the blue channel proportionally to the change of red and green, and then vertically we simply get the brightness, and horizontally — the hue. It is similarly for other pairs.