I originally drew this out with whatever markers I had on my desk and picked pink at first just because I like it, but then the more I thought it through, pink is the perfect color for this, because it is defined by being some mix of red and white. If you take it all the way to either extreme, it's literally not pink anymore. This isn't to say anything about "higher" or "lower" pleasures, but rather that although the instinct is probably to say that darker pink=more pink, that can be debunked easily by pointing out that red is not "more pink" than pink.
A really useful aspect of displaying this issue by image (colors, the vessel analogy, etc) is that you play to the issue of the senses vs intellectual reasoning. Is the glass half-full or half-empty? You're interplaying the senses against the reasoning and having to confront that it's your labeling of the object that gives it it's "moral significance" rather than what you're seeing with your eyes. Are you optimistic and half full, or pessimistic and half-empty? Either way your eyes are reporting exactly the same thing and your mind has to take responsibility for the feeling it generates.
I get the same reaction from the use of colors or shapes. Our eyes tend to act in automatic ways, but we can use a diagram and explanation to force ourselves to confront that our mind is what is doing the labeling. Once we see that we can make progress toward realizing that we ourselves are playing a large part in creating our pleasurable or painful emotions.
All sorts of "optical illusions" probably also have the same value as teaching tools.