I added overrides for highlighting in the ORM diagram very early in the project and could apply those here as well, although I don't think I can get much more contrast than the defaults in this case with a color shifting algorithm. You'd need an entirely different color, not just the Magenta-to-HotPink morph you get now.
Even with an entirely different color, on a densely packed diagram the relationship lines are often partially obscured so even a different color would be hard to trace through. There is a tooltip on these, however, that indicates column names. I could easily extend this to also mention the table name. I think that would be more helpful than a color change. Armed with the opposite table name, you can directly select the shape using the Model Browser.
Does extending the tooltips sound like a reasonable alternative?