In TOS, Uhura was the comms officer, and wore the red of the operations division. In enterprise, the colors seem to be the same as TOS. Archer (captain) and Travis (helmsman) wore gold, the medical staff wore blue, and the engineers, security and tactical wore red. So why would Hoshi not wear red?
According to Memory Alpha Hoshi was a linguist and not an operation officer.
She is a scientist stationed on the ship just like a doctor, meaning she gets a blue uniform.
While Hoshi was the comm officer, she was primarily a translator. Just as Spock was first officer, but was department head of sciences. Since there was more first contact occurring on the mission of NX-01, and the Universal Translator was not as well developed, Hoshi was doing original research. So while a comms officer would normally be someone from the operations division, it wasn't always the case.