The goal was to determine what colors are chosen for a “random” appearance. A (Mario Party) logo that arranges the colors chromatically may include more than just 6 or 7 colors, thereby having two greens, and it becomes confusing to where you draw the line on what color each is defined as. This “arbitrary rule” is kinda related to not including logos that are one color, because similarly including that data would affect determining what the ideal “random” appearance would be. Chromatic or single color do not appear random.
Why though? Seems like an arbitrary thing to exclude. How does that affect the results?