Depends. Tree bark can be a very rich ecosystem, and rain brings out the colors in moss, in lichen, in fungi (as in the first photo above) and in various types of algae and monera. Lichen is the safest bet, though. Some forest ecologists in the UK and in New England have worked out indices of arboreal lichen diversity to help calculate the age of a forest: the more lichen species are present, the older the forest.