Worf was seen still wearing the red colours on Star Trek Nemesis. Was he still a strategic operations officer, or did he become tactical officer again despite wearing red colours?
There is no prime-canonical explanation of Worf's uniform color at that time:
In 2379, Worf rejoined his old crewmates from the Enterprise-E on Earth when he attended William Riker and Deanna Troi's wedding ceremony. Following the Earth wedding and while en route to a second ceremony on Betazed, the second wedding was postponed as the Enterprise-E detected positronic signals from the Kolarin system.
Extended canon offers the following:
Upon his return to the fleet, Admiral William Ross offered Worf the position of First Officer on the USS Titan, under Captain William T. Riker. Worf initially accepted the assignment and was traveling with Riker and Deanna Troi to Betazed to attend their wedding there aboard the Enterprise, (he was also serving as acting chief of security/tactical officer as Christine Vale had taken shore leave on Earth), but after the death of Commander Data in battle with Reman warlord Shinzon, Captain Picard requested Worf remain aboard the Enterprise and Worf accepted, serving as acting first officer. (TNG movie: Star Trek: Nemesis, TNG novel: A Time for War, A Time for Peace)
So, if we accept the input of A Time For War, A Time for Peace, the only licensed work with anything to say about the matter, Worf was in red in anticipation of his role as First Officer of The Titan, while acting in his old role on The Enterprise.