I'm not sure where this started, but I have also heard people say "all multicolor bars have all the colors they just aren't enabled". I'm sure some agencies do "under use" the multi-colors of the bars they order, but Whelen bars don't come with colors that aren't ordered.
I suppose that's not too far a stretch of modern thinking. Tesla makes you pay to unlock power that your car has but is locked behind a software barrier (not that TESLA is a company to look towards for guidance in business practices). One of the luxury car companies was making people pay to subscribe to heated seats that came installed. The idea that a feature can be present but not enabled is rooted in reality.
In the light world some of the cheaper multi color lightheads are easier to sell in their most popular combos and leave one color unused (that's a dealer thing). I had a push bumper I ordered in dual color red/white come with red/white/blue leds in it, but push bars are usually red/white/blue for LE so I can see not bothering to stock just red/white, again a dealer preference. I replaced some Code 3 surface mount LEDs recently and the dealer had no single color ones at all so the user ended up just not using one color.
The idea is plausible, but with Whelen bars they are whatever color combos you or the original owner order. You are free to use or not use the colors they do have, but they don't all come with all the colors.