Note that in 1978 LED technology wasn't nearly as advanced as it is today. Discrete LED emitters were capable of red, orange, yellow, and green ... and I'm not even sure green was widely available yet in 1978. Blue and white (blue with a phosphor coat to re-emit the blue in a broader spectrum) options didn't become available until the mid-to-late 1980s.
I'm not sure what the Phase II Enterprise would have used, but models were
using yellow LEDs for Battlestar Galactica and Buck Rogers at the time. Overlighting it in the studio would have made it pale enough to serve as white illumination.