Ennnhhh - I see a lot of 737s with brown cockpits. Seems to depend on age/era.
As a guess, the introduction of larger and more colorful displays has made a dark gray as "background" preferable (according to some trickcyclist or other). There are lots of people making their living analyzing environmental color and human performance. I think it was a big deal starting with the first nuclear subs - pinks and yellows and such to enliven long times at sea.
NASA's take (skip to page 110 (pdf. pg 113) for the Conclusions - the rest is a review of previous studies) - hue doesn't matter much, but general contrasts of color/tone do have not-fully-definable effects. Related to long-term exposure for space flight.
https://ntrs.nasa.gov/archive/nasa/c...9890006161.pdf
Personally, I find Airbus pale blue-gray nice, as well as lighter brown/beige. Night-lighting may interact with colors, though.