I get the idea of not being able to do it, but there seems to be on in-game-world explanation for it, which is what frustrates me. It's a guy who can walk across perspectives, but not across colors (???). You could do something basic like a plasma barrier appearing when he gets near the transition, or the guy recoloring to the color that he is standing on to indicate being bound to it, then revert back to black when falling through.