The salesman in the showroom told us that the tiles we see hanging on the wall will be 20% lighter when are layed down horizontaly on the floor, so we should take into account when choosing our prefered color.
It's neither strictly true nor a myth. It's an oversimplified (and arbitrarily quantified) suggestion that has some merit. Check color in your room with the lighting it'll actually have. It's more likely that color will vary due to light source color and reflected color from other elements of the room's design than installation orientation, though.