Perhaps for the last two decades or more, the most common colour for piano cabinets has been black; either the high gloss poly or the satin lacquer finishes being the two most popular that we all see today.
Black initially became popular in the early part of the 1900’s. It was thought, at the time, if the instrument on stage was too detailed in cabinetry and colouring, this would distract the audience.
The mindset regarding black was that colouring of the instrument would not detract from the performing artist.
There are, of course, the traditional colourings available, and still produced by a wide variety of piano makers; oak, walnut, and mahogany colours.
At times there are requests for custom colours for piano cabinets. Here are some photos of a piano recently completed in a custom colour called Pacific Ocean blue.
Some of the photos do not show the true colour of the piano as the camera cheats a little according to the lighting available. But this does create an interesting study about how and what kind of lighting affects the picture results.
The last two photos are the true colour of this piano. Here is the link for the photo set.