Why are people amazed when you say, "Tiles A and B are actually the same color -- check for yourself!" but they roll their eyes heir eyes when you say, "There are no squares in this image -- check for yourself!"?

It's about expectations. People expect to be able to take (physical) objects that appear to be different colors, examine them under a variety of contexts, and always perceive them as different. People incorrectly extrapolate that expectation to images, and thus find the fact that removing the context reveals these images to be the same colorRGB surprising. They also expect to be presented with representations, so pointing out the fact that they're looking at a representation seems silly to them.