At our local botanic garden, the bonsai are on stands, that have a frosted glass panel behind each tree. This provides neutral background, against which one can see the branch structure during the day. At night there are lights in the base the glass is mounted in. The light causes the panels to glow, white. This creates stark silhouettes, allowing you to appreciate the bonsai at night. There are also lights on the front edge of the stands, that shine up at the tree. The overall effect is quite beautiful.
As to color of the light. White light looks best to the human eye. I would not worry about "day length" sensitive trees. The lights will likely be low wattage, and majority of trees are not overly sensitive to light issues, or urban landscape trees would be all screwed up by the streetlights. The fact that urban street trees, and city park trees seem fairly healthy, means that they are not overly sensitive to light at night.
You can also control light exposure by simply turning lights off when you are not sitting in the garden. The lights do not need to be on all night.
If you have a wall surrounding one or more sides of your garden, lights aimed up to illuminate the wall at night, will allow you to view silhouettes of trees in front of the wall at night, without having individual flood lights on each tree.
