In TOS, Spock was assigned as both Kirk's First Officer and his Science Officer. Throughout the duration of the series, except for in "Where No Man Has Gone Before", Spock wears a Science Division uniform.
Meanwhile, in TMP, Decker is temporarily assigned as Kirk's First officer (either temporarily or formally?). In addition, after the death of the Commander Sonak, Kirk requests Decker to "double up" as the both his First Officer and the ship's Science Officer.
To be fair, uniform regulations clearly changed a lot between TOS and TMP, so maybe that could account for this seeming aberration. But does anyone have another explanation? One that might be more cohesive with the uniform expectations across the rest of the franchise?