Why is it difficult to know whether two people experience colors in exactly the same way?

In the philosophy of mind, the difficulty in determining whether two people experience colors in the same way is a central problem often illustrated by the "inverted spectrum" puzzle.1

This challenge arises because color experiences are considered qualitative states—or "qualia"—which are intrinsic features of conscious experience. While we can observe physical facts about a person's brain or their outward behavior, these physical data points do not inherently reveal the nature of their internal, subjective experience.21

The symmetry of the color solid suggests that it is theoretically possible for someone to use color vocabulary correctly while having internal experiences that are the reverse of another person's (e.g., seeing "green" when looking at blood). Because we lack direct access to another person's mind, we cannot verify if their internal "red" experience matches our own. This is a specific instance of the broader problem of other minds, which questions how we can justify the belief that others possess conscious mental lives similar to our own.13