The |\ key (and the ~` key) is not a modifier; it is an alpha. That's why it is colored like an alpha. This organizing principle is based on key purpose/functionality, not on an OCD-driven need for color symmetry. I prefer not to let the crazies dictate how we organize our keycaps, tyvm.
Besides, once you graduate to keyboards with more than 60 keys, color symmetry goes right out the window anyway, with the nav cluster weighing down the entire right side with mod-colored keys, and the numpad layout only half-surrounding the numbers (and period) with mod-colored operation keys (plus num lock and Enter). And then there's the alternating color blocks of F-keys, which makes no sense either functionality-wise or color-symmetry-wise. Best to just realize that the standard color/layout scheme we have is based on an ill-conceived IBM design and learn to embrace it since it's not going away any time soon.