We can both detect orange light, but the shrimp can also detect red light mixed with yellow light. We get tricked into also seeing orange, despite no strictly orange light being there.

Think of it like sound. If we heard sound like we see light, we would hear a chord as identical to a tone somewhere in the middle of those notes. Mantis shrimp can see chords of light without any mixing, seeing both colours rather than just a colour somewhere in between.