I can see the difference between the three as well.

Of course, part of the problem here is the same problem that comes up with the Adelson Checker Shadow Illusion: how a human interprets colours or shades depends a great deal on what it surrounding the object you are looking at. I would wager great sums that were you to separate the three objects with something of very different colours, so that they'd be yellowish objects surrounded by something very different, most people wouldn't be able to detect any difference between them at all.