There’s very little practical reason all three of them can’t be in white. Even from an aesthetic, production design, stand point there isn’t much there. From a framing stand point if you’ve got 3 people, and you don’t want them all in white, its the middle person (Sherlock) who should be in another color. If they were all in the same studio, in the same camera shot. And at least one of them (but not all three) had a much darker skin tone there might be a practical concern.
This is some one pulling rank over minutia. Pure and simple.
