tythur,
Sounds like your in laws fell from the same tree as mine.
When I cook a nice, and often expensive, prime rib, all in the family except the in laws like it rare/medium rare and I refuse to ruin it for 2 people who are deathly frightened of a little color. The simplest solution I've found over time is along the lines of Chef JJ's suggestion, though I hate to waste a well prepared jus that's normally served at the table. For a quick, easy, and cheap fix I simmer a quart of beef stock or broth in a large saute pan on a back burner of the stove and add a few healthy dashes of Kitchen Bouquet which contains caramel for coloring. I slice their portion first, throw it in the pan, and continue slicing the remaining portions. When ready to plate and serve, I pull their portions from the pan, and the magic is done. Their slices are dark brown and everyone else has a red/pink slice.
Works every time and they're none the wiser.