When creating the plant life on alien planets, photosynthetic pigments other than chlorophyll can be the predominant molecule to create foliage of colours other than green, as we see on earth but for the minority of plants. But most trees and woody plants have brown bark, other than the few examples like silver birch which has light coloured bark to regulate its temperature.
Some plants have green bark as chlorophyll is present but they are in the minority, although if other photosynthetic pigments are present in the leaves the bark could be that colour but as the majority of plants have brown bark you could assume that on alien planets not all bark will contain photosynthetic pigments so some sort of other coloured bark will likely be present.
Tannins are the molecule in woody bark responsible for giving the brown colour and its purpose is to protect the plant from pest. This makes me wonder if other molecules could replace tannins to carry out the same functions for the plant but would give the bark another colour other than brown?