Decker wore the beige in pre-publicity shots because he was the last person cast and his uniforms weren't ready yet. The beige appears rather gold in that series of pics due to lighting, and it was a prototype of the grey jumpsuit version he used in the movie. In those same shots, Chapel is also in blue/grey, but wears beige in the movie. Spock is missing his black Vulcan collar, and Kirk's uniform has both sleeve stripes and epaulets!
The production notes for TMP say that all crewmembers could have their choice of uniform colour. It's not indicitive of senior vs junior. (Division was shown by the circled circle behind the insignia and, on some styles, epaulets, and rank shown on sleeve stripes or epaulet stripes.) Note that Lieutenant Ilia and the Rhaandarite bridge ensign wear blue/grey, and they are both outranked by Uhura, Sulu and Chapel.